I could try for a thousand years to write the most perfect essay containing every single reason why this pairing is so great, but I'd never even come close. Still, I plan to try! Also, I will be spoiling the entire series, because I can't really discuss a pairing while avoiding touchy areas.
For a start, they're not your typical average pairing. Usually the main couples consist of a guy and a girl who get on each other's nerves, they constantly fight and pretend to hate each other, and they gradually get over this denial and actually admit to a few feelings for each other. That isn't RaielxSizer. On the other hand it isn't complete crack where all we have behind it is "ZOMG THEY WERE ON SCREEN AT THE SAME TIME OTP!!"
RaielxSizer is different, and adorable, and if you're reading this then you probably understand what I'm trying to say. ^_^;

To begin with you have to examine the individual characters. There's Raiel, the self-proclaimed "Ai no Yuusha" (Warrior of Love) who has terrible nosebleeds whenever he's actually around girls. At the beginning he's really only in love with the idea of being in love. He does genuinely care about the people he helps, which we see during the Fife side story, but he's never actually experienced proper feelings of love. So after Sizer kisses him and offers him a random loose feather, he obviously bleeds down to the point of death, and although it's so cute I still passed it off as 'okay, a seriously hot girl just kissed him.'
He has his first Sizer fantasy soon afterwards - which hooked me on the pairing - but still it just seems to be due to hot girls kissing him. It's the same with the second extra story in v8: he loves the idea of the Hot Girl being completely innocent and in need of a heroic rescuer, and him being that heroic rescuer. And although I think that side story is fantastic and amazing, we don't see - and Raiel doesn't see - Sizer as an actual three dimensional character in it.
We get a little hint in v9 when we see Raiel staring in deep thought at Sizer's feather, but the first real evidence of Actual Feelings we see on Raiel's part is his big heroic rescue of her in v10. A heroic rescue of a helpless damsel in distress, what more could Raiel want, right? But he reveals that there's more to it than that. Both he and Sizer are spirit summoners, and thanks to the Valkyries he has actually gained a bit of insight to her. He speaks not of rescuing Helpless Hot Girl but of rescuing Sizer, the suffering girl, the person inside the outer angelic appearance.

For many volumes after Sizer joins the party in v11 we see plenty of attempts on Raiel's part to impress Sizer, or involve her in fantasies. Like a true romantic, he gets horribly depressed when Sizer doesn't play out her part in these fantasies, but he's such an adorable little idiot that we can't help but love him. X3 Still, when Sizer needs guidance, reassurance and understanding, Raiel is there. Hamel and Flute are off in their own problems, Ocarina is away scouting, Trom is hating her and Oboe...is randomly not paying attention to her. But Raiel sits with her, talks to her and tells her about his and Hamel's childhood, which is exactly what Sizer needed at that point in time.
We get our first huge glimpse of 'hey, maybe he actually means all this love stuff' in v18 when he comes to Sizer after she and Trom have fought. He listens to her earnestly, genuinely tries to comfort her, and then takes a sword in the gut to protect her when Trom comes back for more. Once again he reveals to us that he understands Sizer's pain, and wants to help her for much deeper reasons than just because it's the typical fairytale thing to do. Sizer 'kisses' him in order to draw the blood from his lungs, and the look on his face as she does so isn't 'hell yes, a hot chick is kissing me!' but complete shock because he didn't actually expect any affection from this broken girl. And so he's put out of action for several volumes, but even while injured he continues to worry about her. With good reason too, because after recovering from his wounds he rejoins the party only to find out that Sizer, who was finally starting to heal from the emotional scars of her childhood, has had her soul stolen and is now animated only by a desire to kill. He meets Kuro Sizer for the first time in v27, and the reality of this heartless killer hits him square in the face.
Does he snap his fingers and say 'oh well, there are plenty more fish in the sea'? Does he secretly resign himself to the fact that the Sizer he knew is gone, like Horn and some of the others? No, he hops right onto Hi no Tori-kun and goes flying after her, desperate to bring her back. Despite seeing what Kuro Sizer is and what she will do he never for a second gives up hope of bringing Sizer back. And then, as you can see in the twenty-nine pages of scans in v28, he goes through hell, he keeps going when even her own brother and lifelong friend have given up in order to save her. To quote my friend Celetina, who really says it best: "When he talked earlier about how he would do anything for Sizer-san, how his love for her was the most powerful thing in the world, we rolled our eyes and said 'Oh, Raiel, you're just a hopeless romantic.' Now in volume 28 we realize holy shit, he actually meant it, and that's pretty damn awesome."

In v29 he gets to come to her aid several more times, but by this stage it's pretty damn clear that this is no longer, in any way, shape or form about him wanting to be a big macho hero, declaring this and that to be in the name of love. He does it all for Sizer, to see her smile, to heal her pains, to help her realise that it's okay to stop hating herself and wanting to live. His slap brings her out of her suicidal insanity, and his kind words help her to laugh and allow herself to accept love after the most important person in her life has passed away. And with Sizer, these are not feats to be taken lightly!
Afterwards, when she is actually willing to accept his feelings, he is still primarily concerned with just being there to support her in any way that he can, whenever he can. He doesn't even declare himself the Warrior of Love anymore, because his heart has found its purpose and it won't budge from there ever again.

And now Sizer, the real brick wall in the relationship, and also the one who brings it to reality. While Raiel is always completely open about his feelings for her, she doesn't even understand what those feelings are.
Sizer is, mentally and emotionally, the most messed up in the party. Having been raised by the mazoku she doesn't understand such things as 'love' and 'affection'. It's not like many characters who dislike the whole 'love' idea or refuse to acknowledge it, Sizer honestly has no clue what it's about, or how to recognise it. That's really the key to Sizer and her part in this relationship - she's just a complete innocent and doesn't understand the slightest thing about these emotions. And yet, despite not understanding them she deeply craves them, and after joining the party she spends a lot of time observing her new friends and trying to make heads or tails out of the way they act. She quickly attaches herself to Flute who offers her support and friendship, but Raiel unfortunately goes about reaching out to Sizer in the wrong way. He tries to impress her with big romantic gestures as if she were a typical teenage girl, not really realising that he's actually confusing her even more than she was before. She passes these attempts of Raiel off as random idiocy, and at first only goes to him in order to find out about her brother's past. However, as she listens to him talk about Hamel and Pandora - and Pandora's frequent attempts to kill him - she begins to realise that this confusing idiot is actually a lot deeper than she had originally thought, and begins to see the affection and compassion that Raiel is capable of through his stories. She doesn't notice it at first, but she begins to get more comfortable with the group even with Ocarina gone, and starts to find a bit of a place for herself there. Still, she generally ignores Raiel as the guy who keeps apparently attempting to confuse the hell out of her and focuses more on Flute, as the original source of happiness that she found after leaving the mazoku.

Unfortunately, just as Sizer's so close to healing and knowing happiness, Trom decides to attack her nerve and Vocal appears in a huge human killing spree and this sends her spiraling back into guilt and depression over her past. In a way this sends her back to square one emotionally, and this time it's not Flute, but Raiel who's the first to come and offer her the support she needs. She talks to him, revealing hidden parts of herself, but is interrupted as Trom returns for round two. The guilt is eating her up so badly that she tells Trom to stab her, to avenge his parents and destroy the monster who helped in their murder. As Trom lunges out blindly with his sword Raiel jumps in front of it and takes the blade in his stomach. Again, this results in complete shock and confusion for Sizer. Why would anyone protect an inhuman monster like her? Surely someone as vile and sinful as her doesn't deserve to be saved. Just who is this man who would jump in front of a sword in order to protect someone like her?
Then Raiel starts to speak, encouraging Sizer and telling her that she has friends and people who care about her. Sizer then leaps down to 'kiss' him, and I'll let you make up your own minds about how truthful she was in her 'removing blood from throat' explanation ;D. She says to Raiel, directly to him and not Trom, that he's kind but that she's not wanted in the party and so she has to leave. Call me a crazy fangirl, but the way she tells Raiel that he's kind and the tearful sad smile she has on her face as she says it (and as she flies away) make it seem as though what she's really saying is "I know you believe in me, I know that for some reason you want me here, but I just can't stay."
But although she claims this to be her reason for leaving, we find out later when she's talking to Ocarina that the real reason she left is because she wants to sever her last tie to her past be defeating Vocal, who so reminds her of her previous self. She says to Ocarina, in what is a huge moment for Sizer, "I want to live". The suicidal girl of the past has really grown since meeting with Hamel and the rest, and she finally feels she's worthy of living. She wants to live, she wants a future and she wants to spend more time with the friends who care about her. I'll say that again just in case you missed the gravity of the situation: Sizer has finally begun to believe, for the first time in her life, that she deserves to live.

Unfortunately for her Vocal shows up in the middle of this big scene, and so the two of them fight ferociously. It's really no competition between them though, and Vocal completely - for lack of a better word - PWNS Sizer. But because he's bored and the real person he's after is Flute he ends up leaving her, and although Sizer is seriously wounded and bleeding all over, she pulls herself up with the help of her scythe and determines to return and save her friends no matter what it takes.
This ends up leading her to Dal Segno, Trom in tow, in which she fights again - but this time it's not for her, it's all for those she cares about. And she fights valiantly, but long story short her soul gets ripped out of her body, stuck into a sword and a dark sphere from Vocal gets shoved into her soulless body instead. Hello Kuro Sizer!

We're never completely sure just how aware Sizer is of what's going on while in the sword, but it is clear that she at least vaguely knows what's going on. However aware she is, she eventually finds herself back in her real body missing an arm and a leg, but also with a bloody and half dead Raiel in her arms. Once again this strange, confusing man has put his life on the line in order to save her. But this time there's no expression of surprise, or confusion on her face. She just hugs him very tightly. Not only that, but she notices the feather in his hat which he's kept ever since their first meeting. And so, like Raiel's kept it all this time as a constant Sizer reminder she now takes it as her own Raiel reminder when she goes off to fight.

However, the death of Ocarina shatters Sizer and for an instant she raves madly and even tries to kill herself, but it's Raiel slap that returns her to her senses. And later while she's mourning at Ocarina's grave Raiel comes to her, offering comforting words and Sizer once again finds herself able to open up to him. And although she would never have thought it possible straight after Ocarina's death, Raiel helps her to smile again. In fact, it's the first time Sizer has smiled, really happily smiled in the entire series. It takes her twenty-nine volumes, but she finally manages to realise that it's okay to want to live, and, hey, maybe this confusing dork actually has...feelings for her! And just maybe she has similar feelings in return. ^_^ This continues nicely into v31 where she goes out to find him at night and the two talk for a while, but then just sit silently, completely comfortable together under the moonlight.
Compare this to the Sizer of earlier volumes. She never would have gone to Raiel like that or opened up to him, and she certainly wouldn't have felt comfortable sitting with him like that, even without holding his arm! And now that she's finally worked out her issues with the past, and has finally begun to understand what these crazy emotions are she becomes completely devoted to him. To the point of a jealous rage when Fife returns in v32. ;D

Perhaps one of the most beautiful, but also heart-wrenching RaielxSizer scenes is when Sizer fights Chestra in volume 37. She's clearly no match for him, even in her current state, and Raiel distracts Chestra just long enough with a Hi no Tori attack to leap to Sizer's side. While holding Sizer he yells at Chestra, defending her once again as he did in v10 when she was helpless under Guitar's power. Although Raiel has twice come to Sizer's rescue, he doesn't just save her and leave her at that. When he's yelling in her defense at the enemy he also encourages Sizer, and gives her the strength to believe that she can fight back! She listens to his words, smiles and snuggles close while thinking "Raiel...I...love you, really." (though I haven't actually read through all of the manga, since I don't understand much, I believe this is the first time she's actually used the L word to Raiel. Correct me if I'm wrong though)
Chestra then attacks Raiel instead with his extendo-fingernails of doom (think Lust from Fullmetal Alchemist) and she screams, and screams, but then leaps at Chestra with her scythe crying "GET YOUR HANDS OFF MY RAIEL!" and...Chestra uses Raiel as a body shield. Her newfound belief in herself turns out to be a double-edged sword, because once the person who gave her the belief is almost dead on the ground before her she completely shatters, screaming at him to please, please not die. She knows Chestra is behind her. She knows he won't cease his attacks there. And yet she doesn't move, even as his fingers strike through her neck, and then she uses the last of her strength to reach out and take his hand. They lie like that, Sizer on top of him holding his hand, and she would have been happy to go with him rather than fighting without her Raiel by her side ever again. It makes her sounds weak, that she would just give up without fighting to the very last breath in her body. She is weak, but then love makes even the best of us weak, doesn't it? ^_^ If there had been any, any doubt at all to the sincerity of this relationship beforehand, here is the final proof, don't ever claim to us that RxS is a stupid relationship with little ground beneath it ever again.

But the best part is afterwards. After Flute has healed them and they've woken up (still holding hands, mind X3) and after the battle against Chestra has finally been won, Sizer gives Raiel the biggest, happiest hug ever and Raiel....of course has a nosebleed. XD Haha, after all this time and even after they've grown emotionally so much, Raiel is still Raiel. And from what I've gathered from the epilogue it takes him another almost ten years to finally get over this problem. ;D
The first thing I noticed in the epilogue is that Raiel calls out to Sizer, but for the first time that we've seen he doesn't add a 'san' to the end. Even something small like that shows how comfortable they are together, and why wouldn't they be considering Sizer's just given birth to a beautiful baby girl. X3!! Raiel's still as adorably dorky as ever and thanks Sizer madly for the child, leading to a nose bleed. Some things never change. ^_^ And, as they say, they end up living happily ever after.

Raiel and Sizer are not only great as a couple, they're great together. They're good for each other, and they both grow so much emotionally through their interaction. The relationship, while clumsy for most of the series, is still nevertheless always adorable and certainly different from what we're used to seeing in the other anime/manga pairings. And so, because I've always hated ending essays (real or not) I'm just going to finish this up with: RaielxSizer Kicks. Much. Ass! WOO!

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Holy hell that was long, and pointless. Why was it so much harder for me to write about Sizer, considering she's my favourite character in the series and there's so much more on her to talk about? Bah, well, I'm not going to pretend I'm good at writing or structuring essays; this was all set out horribly and I myself cringe at it. XD But hey, I tried. I'm just happy that it's finally finished!