The 2nd Child
Asuka is referred to as the "2nd Child" - the 2nd person found who had the ability to pilot an Eva. Asuka was around 7-years-old* when she was identified as a possible Eva pilot, and I believe began training immediately after identification. She was trained in the 3rd Branch of NERV in Germany. Her first battle took place at the age of 14 over (and under) the sea!

Interesting Info
Asuka is an American citizen with mixed Japanese-German blood - 3/4 German, 1/4 Japanese. She frequently inserts German phrases when she speaks.
Asuka is a genius, but she does just as bad as Shinji in school because she 's not very good in reading Japanese! Therefore she has trouble understanding the questions. Back in Germany, even though she's only 14, she had already graduated from college! From when she arrived, Asuka had a higher synchronization ratio with her Eva than any of the other Children until episode #16, when Shinji easily surpassed the other Children in the harmonics tests.

There are a lot of interesting things about Unit 02. Just to beggin with, while Shinji and Rei's Evas are purple and blue, Asuka's Eva is red. ("That's really an interesting color. I didn't know it was red" - Shinji).

"That's not all that's different about Unit 02. Unit Zero and One were the prototype and test types, respectively ... MY Unit 02 is the world's first real Evangelion. Created for actual combat conditions, it's the final production model!"

Alright, as Asuka summed it Unit 02 is the first combat designed Eva. It's also the only Eva with a four-eye design. It's components were designed and manufactured in Japan, and final assembly and activation were done in Germany. Unit 02 is also the first Eva to fight under water and also to be operated by 2 pilots, and it happened at the same time! How did that happen? Well, Asuka was rather jealous of Shinji for being the "great 3rd Child," but Asuka knew that she's more highly trained than Shinji and therefore a better pilot. So, when the Pacific Ocean fleet that was delivering Asuka and Unit-02 to Japan was attacked, she decided she wanted Shinji (interesting he had to use HER Plug-suit!!) in the cockpit with her during her debut battle so he can see how great she is. But they ran into a bit of trouble when they fell into the ocean (the first under water battle!), and both Asuka and Shinji had to work together to win ("You're the famous 3rd Child aren't you! Do something!").

"Oh no you don't! The only way you're using my Unit 02 is over my dead body!". Asuka considers Unit 02 as the best Eva and herself as the best pilot. She's very proud of her Eva, and won't stand for anyone else piloting it (which brought on some hilarious scenes and lines in "Asuka Strikes!"). She especially abhors the thought of Rei using Unit 02. Just a little note, in case you didn't notice. Asuka's Eva is red, Asuka has red hair, wears a red flight suit (shown on the left), and her Interface Headsets, which she uses as hairclips in her everyday attire, are also red. Asuka is a bit hot-tempered and also the most aggressive pilot, and the color red signifies this. That very clear clear, huh?

Piloting Unit-02

Asuka is able to pilot Unit-02 using a combination of thought and mechanical control and feedback systems. Asuka's headgear, the electrodes on her head, are sort of thought transmitters that amplifies and sends the signals of her brain to her Eva. As with all the Evas, only a special kind of person is able to "synchronize" with these giant biomechanical weapons, and it doesn't necessarily mean that if a Child can synchronize with one Eva that he/she can synchronize with all the other Evas as well.
In the cockpit, Asuka is submerged in a fluid called LCL ("Link Connect Liquid", Currently, a super-oxygenated silicon liquid such as this is being researched by the U.S. Navy). Her blood is oxygenated directly as the LCL enters her body (think a baby in a mother's womb). During battle, Asuka is not only synchronized with her Eva by thought patterns, but her nerve impulses are connected to her Eva as well. In other words, she can feel what the Eva is feeling physically (well, the Eva isn't exactly "feeling" anything, just Asuka).
In Unit-02, Asuka is located in the "entry plug" (the cockpit), and the entry plug is located somewhere in the base of the neck of her Eva. But Asuka's point -of-view during battles is where you expect it to be - from Unit-02's "eyes." Unit-02's eyes are some sort of a camera that projects the images to the walls of the entry plug. When Asuka is sitting in the entry plug, she can look all around her and see everything that's going on.

Asuka: "You jerk! What are you doing on my Unit-02!"
Shinji: "What are you talking about? You're the one who fell on me!"
Asuka: "You're the one who lost time first, [German: stupid]! You're always so slow. What were you doing staying up so late last night?"
Shinji: "Mental training for today's battle!"
Asuka: "Oh, mental training, I just bet. Liar! I know you tried to kiss me while I was sleeping!"
Shinji: "You CHEAT! You were awake?!"
Asuka: "Oh my gosh, I was just kidding! Did you really kiss me? Eeeuuuwww!!"
Shinji: "No, I didn't I stopped, I . . . "
Asuka: "[German: something like "perverted . . . something"]" I can't believe...
Commander Fuyutski: (In headquarters. Asuka and Shinji still arguing in the background) "Those kids are embarrassing us again."

Asuka Past

Asuka's tragic story began when she was very little. Her mother, Kyoko Zepplin Sohryu, was a technician engineer and a test pilot during the early production days of Eva. After an accident during a test activation, Asuka's mother went insane. She would only recognize a doll as her daughter and referred to the real Asuka as "that girl over there," while Asuka stared with angry wide eyes at her mother and the doll through the window of a hospital room.

"Asuka darling, mama cooked your favorite for you. Oh no, you musn't complain about what you like and dislike, or that girl over there will laugh at you."

Through her insane words and actions, Asuka's mother was trying to make-up for lost time with her daughter, as she was filled with guilt for devoting her entire life to her reasearch instead of taking care of Asuka. Asuka's father shortly remarried, neglecting both his daughter and his insane wife, which crushed little Asuka.

"Mama, they chose me! I'm an elite pilot now. I am the best in the whole world! I must keep this a secret, but I'll only tell you mama. Everybody's so nice to me now. I don't feel lonely anymore. I'm okay now, even without papa. Look at me, look at me mama. LOOK MAMA!"

Shortly after she was identified as the 2nd Child, little Asuka ran home, excited to tell her mother the great news. This was a reason for her mother to pay attention to her now instead of the doll, she was sure of it. The things she wanted to tell her mother rushed through her head, and as she opened the door open, Asuka yelled out "Look Mama!"...
Asuka still had the smile on her face as she stared unmoving at her mother hanging from the ceiling with a rope around her neck. Asuka's mother had commited suicide. (Personal Comment: Gosh, this must be horrible...)
The events of her childhood would prove to shape the rest of Asuka's life. Little Asuka promised herself that she would never cry again. From then on, Asuka made sure she was the best at everything. She graduated college at a very early age, and she cherished her title as the "2nd Child," as this gave her importance, identity, and control over her life. At the same time locking away her sad memories. She devoted her whole life to being an Eva pilot and used her skills as a pilot to measure her self-worth.
When the anticipated enemy attacks began happening in 2015, 14-year-old Asuka and Unit 02 are shipped (together with the ENTIRE U.N. Fleet) transferred to Japan. There Asuka met the other Eva pilots. Asuka prided herself with having the highest sync ratio with her Eva than any of the other pilots. It was clear that Asuka was the only one who enjoyed her work, and considering the burden of being a pilot, this is a bit perplexing to those not familiar with Asuka's past. She was a happy girl and always optimistic. She hid her past not only from the people around her but from herself as well.
A series of events would trigger Asuka's downfall. First, the 3rd Child, Shinji Ikari, surpassed Asuka in sync testing in episode #16. Then during a battle towards the end of the series, Asuka suffered her first lost. She felt very helpless as the enemy was much stronger than she was, and Shinji had to take over and defeat the enemy. This hurt Asuka's pride immensely, and her sync ratio began to fall.

"When I didn't win with my Eva, I lost whatever value I might have had. I hate . . . everything. And I hate myself the most. I don't care about anything anymore. There's no point."

In Asuka's next battle, the enemy attacked her mind, defiling her sad memories that she had tried hard to forget. During this battle, Commander Ikari refused to send Unit 01 (Shinji) to save Asuka, despite Shinji's pleadings, for fear of the enemy getting a hold of Unit-01. Unit-00, eventually defeated the enemy. Asuka lost all confidence in herself.

Misato: "What are you doing? Asuka move out now! What's wrong with her?!"
Maya: "She can't sync! Her sync ratio is under 10%!"

When another enemy attacked, Unit 00 was in major trouble. Asuka was sent to "take some of the heat off Rei," but as Unit 02 was launched, Asuka found that she couldn't control Unit 02 anymore. So instead the Commander, despite the obvious risk of exposing Unit-01 to the enemy again, ordered that Unit 01 (Shinji) be sent to help Rei anyway. As Unit 02 was being retrieved, Asuka said bitterly, "Why for her, am I so worthless that you couldn't do this for me?"

Asuka's entire life was based on being an Eva pilot, so when she lost this ability, she lost her mind as well. She ran away, only to be brought back by NERV's Secret Security, but by the time they brought her back, she stared with empty eyes. She was placed in the hospital, and the TV series ended with her in a catatonic state, unaware of all her surroundings.

It's also worth pointing out that during the battle with Shinji vs. the 5th Child, the 5th Child said that the reason he was able to synchronize with Unit 02 was because Unit 02's soul was in hiding, meaning Asuka had lost control of it.