Because sometimes I just have to remind everyone that I have no life.
Anyway, I finally figured out my argument for why Lenalee is as important in the series as Allen - and also why sometimes other arguments don't make sense to me. After reading the first few volumes of DGM (again), I finally realized why everyone has been saying things like that. And you'd be right. Kolox, she WAS like Shinobu and Nodoka, and Reinard, she WAS just a boring character who could easily have died. That's how it is for like... six volumes. The reason I argue that she isn't is because she has a totally different personality now - as does Allen. For all of you (again, KOLOX) wondering why no one has really changed as a character in the series, I can only say that I'll try and show you what I think here. Allen and Lenalee, of all the other characters, HAVE changed quite a bit since the beginning of the series.
Once upon a time, Allen was boring. He had no real personality. His worries about Cross and debt were half-hearted, and his fights with Kanda were just pathetic. He had no real ambition. Even I can't really see that character as being anything but boring and transparent (although without filthy hands! =P). Yet that's not the character that we're currently following in the Ark. The Allen with the power-up has a much stronger personality. He's far more serious, most of all. He's willing to threaten people, to injure (if not kill, just yet - although it would seem that not killing Tiki was more the result of Allen's realization that he has another main goal rather than not wanting to kill humans) them, and to actually fight people. At the same time, that change is noticeable in his interactions with his friends, too. Fights with Kanda are more amusing, and sometimes they involve Lavi. When he speaks to Lenalee, he actually looks happier and as if he really does care about her rather than the bored Allen of the first few volumes.
I thought this change happened around volume 7, when he died and had to go to the Asia branch. That's when he started showing emotions, probably because it was a serious time with serious scenes and all that. He actually felt things, and showed that - regret, anger (both serious and comical), depression... and even if those aren't happy emotions, they were still a big improvement over what he'd shown in the past. I guess either Hoshino had been planning that, or she just decided around that time to change her characters that way. Well, anyway, that's where and how his character changes. Seriously, go back and read some earlier chapters, and then later ones if you don't believe me. Allen barely acts the same at all.
As for Lenalee, she follows a similar pattern. She barely appeared in the first two volumes except as a supporting character; it was surprising to find that she was an Exorcist, since she had such a quiet personality. I mean, she's serving TEA to people... >_>;; Honestly, I can't think of anything to say about her at that time. She's still boring through the next few books, all the way up to... yup, volume 7. As soon as Allen dies, she becomes seriously depressed, and then seems to also gain a determination that we hadn't seen in anyone previously. Of course, now, she's the most emotional character in the entire manga. A lot of people don't like that side of her, but would you really prefer the timid Lenalee from the beginning who served people tea and didn't really seem to care about her friends all that much (she even was kind of cold to Allen in volume 3)? She has personality, and it really makes a difference!
So as for examples: In volume 3, she shows that yes, she has a spine, and she smacks Allen (which really doesn't seem like a friendship thing, but hey...). I always thought that it meant her character was consistent, but the way it was written implies that the slap was a big deal and that she wouldn't do that kind of thing very often. It was serious. Of course, that's very different from her character 10 volumes later, where she punches even her friends in the head when she gets angry at them. Allen even comments that she's become more aggressive lately (chapter 123). And it's true. Would the old Lenalee have cried so much over her friends when something happened to them - or at least, would it have seemed to be part of her character? Additionally, we've seen at least one change that was definitely inspired by events in the manga; pre-Allen-death Lenalee would never have gone to save her friends when they were in trouble (she sent Lavi to find Allen in volume 4, and left Allen to find help in volume 6), while Ark-Lenalee was willing to break away from the group and go back and find Kanda and Krory (Allen had to convince her to stay with them). It's likely that this is because of events in volume 6; Lenalee felt that she could have done something to save Allen if she hadn't left him, or gone back, so isn't making that mistake again.
Okay, so they've both gone through changes - and of all the characters in the manga, I can only think of a few who have changed that much. Krory and Miranda have, slightly, but those changes come from people accepting them; that had never happened before. Their overall personalities still aren't that different when you consider that influence. (Lenalee and Allen obviously haven't changed because of that, since they've always been "accepted" in society - or at least by some others.) Road has also changed a little, but since we haven't seen her develop, it's more likely that her slight lack of bloodthirstiness and desire to murder are just a change by Hoshino for the plot. Those kinds of changes happen to protagonists - the heroes - the main characters of the story. So while Lenalee may not have started off as one, and Allen was a pathetic one, they've grown into those roles. That could also be why Lenalee never had any involvement in interviews or anything else between the pages until later in the story; she's included in those little drawings and whatever (and on a Jump magazine cover =P).
That's also why I like short-haired Lenalee. Yes, she cries more than she used to, but she has a much stronger personality with more emotions (even ones that don't involve being sad). Even if she's physically weakest, she's still quite strong, mentally. That's why she needs a power-up: to show the strength that she's gained just as Allen needed one himself. In Japan, girls will cut their hair short after an event like breaking up with a boyfriend; it represents a change in their life, and often, in manga, these girls will have stronger personalities. Lenalee didn't get dumped or anything, but it could be a sign of her growing independence - she thought of Komui when she died, with her as a little girl, and her haircut could mean that she won't be as dependent on him as she used to be. (That could also be a sign of romance. =P)
And speaking of romance, is it any coincidence that the majority of the romance in this series is happening now? With the exception of talk rooms (although the one in VOLUME SEVEN had quite a few hints), there really wasn't anything before, yeah, volume 7 that really showed any romance. It's only now that they've been reunited that any hand-holding and threats of Komui and even general closeness and emotional dependence are occurring. =P It also makes me think of the flashback in volume 8, which is meant to be set in... about volume 5, I think, but features a more emotional Lenalee than the one who existed then. Similarly, Krory's flashback in volume 12 shows a similar situation; Lenalee may be mild and sweet even there, but she shows more personality in doing that than in previous books. Additionally (on the topic of romance), pre-volume 7 Lenalee was quite aggressive and cold to Allen at times (no, not being tsundere or whatever you call it, just mean), while Lenalee in the Ark shows affection even while beating her friends up. xD I honestly can't imagine her saying nasty things, or criticizing them as easily as she used to.
In summary, Allen and Lenalee have both changed dramatically, which makes the manga much better than it used to be~ Without those changes, we wouldn't have any of the character interactions in recent chapters, and there wouldn't be much potential for the future. I'm really happy with these changes, and I'm looking forward to seeing what events in the manga will occur because of them - such as Komui's reaction to Lenalee's changes. ^_^;; And if any of you have read all this, thank you very much!