@ Magnolia Grandiflora: You're right about everything you say, namely that...
a) RE characters are generally underdeveloped.
... But for me, most of the other characters still have a bit of a personality. When I see Jill, I basically see a classic Mary Sue: She does what's right because it's right (and nothing else), she never fails (at least not as horribly as other characters), everyone likes her (the good guys at least) - she's just, you know, perfect. And while most of these traits apply to most other good characters too, Jill is the only one who doesn't have anything else to offer. At least give her some flaws to make her more human! Leon can be cold and a smartass at times when he shouldn't be, Ada has a chronic backstabbing disorder, Steve is an annoying emo brat, Chris is an unpredictable hothead, same goes for Claire, although to a lesser degree... And then we have Jill, the angel who is decidedly above all that.
I'm not omniscient, I don't claim that everything I say is right. So maybe there's more to her and I just don't see it. But at the moment, all I see in her is wasted potential while I see at least some deepness and character development in everyone else.
b) Jill is not the only character with an unhealthy fixation on another character.
... Oh boy! Claire is a very prominent Chris stalker too (as you pointed out), Piers spends half a year looking for him as if the BSAA was too poor to afford more than one captain, then there's Wesker's obsession with him, Leon and Ada seem completely unable to exist without each other, and the list goes on and on. What I mean by "She's all Chris Chris Chris" is not that she literally mentions or thinks about him all the time, but that her right to exist seems to depend on him, which is not her fault, but Capcom's. And I don't think Chris was willing to let the whole world die once he found her in RE5, I always interpreted his "Yeah, but..." as a "Yeah, but you should come with us" / "Yeah, but let me first arrange help for you" / ... something like that. He wouldn't have said: "Yeah, but now that we're together again, the rest of the world doesn't matter anymore."
c) There was a time (RE3) when Jill got along perfectly well without Chris.
... Yes, and as I mentioned in my earlier post, that was the time when Jill as a character "worked" for me. I actually liked Jill in RE3, her personality seemed different, more present, more believable. If taking Chris out of the game has that effect on her, Capcom should do it more often, even though the two of them being in the same game is not at all a bad thing as long as they have other things to do than staring at each other the whole time. (But Revelations will not be a step forward in this respect, I'm afraid.)
d) The fact that I like Chris while I dislike Jill may confuse other people.
... I haven't always disliked Jill. In fact, she was the very first RE character I've known, and I was quite a fan of her back in 2000, or whenever I played the game for the first time. That was also the time when I thought of RE as nothing but a zombie shooter with some puzzles. Then came the looong break - I only started playing RE again in 2010, and that was when I started digging deeper and actually acknowledging the story and the characters. And I realised that Jill (and Leon too) was not as great as I had thought ten years prior. By that time, however, Chris had already evolved a bit, and the next game that came out (RE6) clearly showed me his not so perfect human side. I'm still waiting for that to happen to Jill.
Speaking of Chris: His problem back then was not the lack of a personality, but the game's failure to show it. While it said in the manual of the original RE1 that he's aggressive and bad with authorities and likes to show off in front of women, the game shows us a Chris that is kind to everyone except the bad guys, does whatever his superior (Wesker) says without a second thought, and treats men and women equally instead of patronising the latter.
Sorry for this long off-topic essay, but I hope it helps to make my opinion on Jill (and Chris) more comprehensible. If not, just ask or write me a PM, I'm willing to explain everything I say if it's confusing.