[Warning: There might be unintentional spoilers in this section. Also, some information here may seem vague if the reader has not viewed Trigun extensively.]

Lady in brown suspenders and a tie
Say hello to Millie, the resident giant-of-a-woman-insurance-lady in the incredible series we call Trigun. Armed with her towering physique, an equally imposing weapon and an ever-present childlike smile, this character plays the important, though under-appreciated role of youngest sister, mother, construction worker, window cleaner, restback, friend, people-relations officer and protégé all rolled in one.

Clad in her trademark yellow poncho, tie, suspenders and white/brown, long-sleeved business outfit, Millie is often seen trapsizing about with her senpai/superior, Meryl Stryfe.

My senpai and I
It would be a great understatement if I said that Millie is a perfect partner for Meryl. Whereas the latter is prone and HABITUALLY falls into a fit and turns into a nervous wreck when things fall out of hand, the former acts as the sedative of sorts. Afterall, Meryl is one person who ultimately lands the task of chasing the "hoodlum" Vash, getting the job-related tasks (paperwork and the 24 hour surveillance on Vash) done on time, barging into offices of the people with responsibility to try and talk to them, announce to whole towns how "terrible" Vash is, and generally bitch around trying to keep things in order.
Millie, however, is the ultimate stopper for Meryl's impending heart attack and nervous breakdown. Oftentimes she is the patient one who sits through a barrage of Meryl angst and whining, the one who shakes Meryl back into the reality of the problem at hand; indeed, she is one of the greatest means for Meryl to hang onto her sanity - considering how many misadventures they experience.
Meryl sees to it that the job gets done; Millie sees to it that Meryl is sane after the job gets done.
Another way to look at it is that Meryl acts as the brain and logic of the two, with Millie being the heart of this unique partnership. (Needless to say, it is quite apparent that the two act more like sisters than strictly a superior-subordinate pair.)

(Now, onto the main subject of this site: Millie! Apparently, you cannot avoid talking a bit about Meryl when talking about Millie... or vice-versa.)

Happy-Millie-Sense
Millie is a huge, happy child. She is usually the last one to frown in the face of adversity and the first one to smile when the smallest of things happens - so much that she even appears detached from this plane and living in an eternally-happy-fantasy-"Millie" land.
The youngest in a family of 10 children, Millie acts and behaves very much like THE youngest sibling of everybody in the Trigun party. Tactless, clueless, innocent and undoubtedly the most affectionate and unabashedly caring of the whole lot, she also has an uncanny knack for being able to befriend and relate to almost everybody she meets (kids AND adults alike). High EQ, as they say. Also, Millie, though not as "bright" in the usual sense, is incidentally very much always the first one to notice and to take note of the not-so-obvious things. Perhaps the greatest proof of this was that she immediately got the fact that the man in red is actually the real Vash Stampede while everybody else doubted who was who. The notion that she is quite "slow" stems from her seeming inability to connect the not-so-obvious observations she gets with the very obvious things many other people notice immediately.

Well, that's one role Meryl plays for her.
As Vash said: She (Millie) doesn't know how smart she really is.

Beyond this, Millie also has the highest regard for a good and respectable family life. She feels strongly for the sanctity of a whole family as well as respecting parents. In fact, the times when she goes over the line and acts too recklessly usually happens when the values and beliefs she has about family are blatantly disregarded.

Oh, did I forget to mention that Millie also has tendency to get herself drunk?

Huh?
The first thing that came to my mind when I first watched Trigun and saw Millie was:

"Whoa, this girl is HUGE."

And, as I discovered as I grew more familiar with Trigun, her size is matched with a strength befitting her mass. Oh no, she doesn't slap or pull hair or go screaming off but hitting nobody. Millie pummels and can literally make pancakes out of an opponent's face if she connects. Oh well, she (spoiler alert) DID punch Vash squarely in the face (and considering how physically strong Vash is, the fact that her punch managed to twist his face 90 degrees to the right IS something) and she nearly KO'ed that son in Episode 14 if he had not been able to dodge her fury. Then of course, she literally and effortlessly hefted a hysterical Meryl in Episode 16 when they had to escape the blast of Vash's Angel Arm. Later still, after she falls for Wolfwood and he eventually dies, she runs around with his Cross Punisher until she gives it to Vash for his final duel with Knives.
Then we realize that she manages to lug this huge Stungun with her as she travels in the desert - WITHOUT any apparent difficulty at that.

[After reading through this profile, I realized that it may have been vague and somewhat hard to understand. (Hey, I did write this in one sitting.) Nevertheless, I do hope that I have been able to adequately express my point.]


In the End...
In the end, I guess what I wanted to say is that Millie is a huge, very happy, and uniquely perceptive child in the Trigun universe. This, plus the fact that she is very powerful (lugging an equally huge weapon) and is family-oriented gives her an identity all her own.

An identity all to complicated for words and write-ups like this! =)
(Provided, of course, that she doesn't go wild while she's drunk.)