The other day Haru made a comment to me about how possibly Chisame liked Mello so much because of some sense of schadenfreude. I thought about it a little and agreed, and then I really thought about it, and I came up with Stuff. Wanky character analysis Stuff. So enter the fangeek zone at your own risk.
At her core, Chisame has two motivations and two motivations alone: insecurity and trainwreck syndrome. These two things really explain everything she does--I'd say both in the canon and in CFUD.
The insecurity is obvious. It's probably the most blatant aspect of her entire character as Negima gives it to us. Chisame dresses up in private to win the adoration of men, but can't handle people in real life; it takes one kind word from Negi to start her downward spiral into crush. One manga chapter of kindness and then 7 volumes later (which she barely appears in) we find out she likes him. Her interaction with him is telling--in private she's all about gloating over her beauty and fame, but as soon as someone else acknowledges any of it, she freezes up and goes completely red. She's not used to REAL compliments, and believes she doesn't really deserve them. When Makie and Ayaka join the cosplay contest in volume 11, Chisame tries to quit, convinced she'll lose to them and be crushed--despite her experience, preparation and superior costume. Again, she doesn't really think she deserves to win, even on her own fairly obvious merits (which she doesn't believe she has).
The trainwreck syndrome isn't something she's consciously aware of. In the manga, we see her over and over again proclaiming how much she dislikes the events going on around her and how much she hates associating with the freaks of her class. But--she's THERE. Every time, when there's no other motivation for her to be hanging around, she's THERE in the midst of the carnage. She agrees to go on the picnic in volume 2; she's at the "cheer up Negi by molesting him in the bath" party in volume 3; she VOLUNTEERS for the hotel kissing game in volume 5 (really telling); she's in Cinema Town with her group in volume 6; she's helping plan the school festival and giving cosplay advice in volume 9; she's at the battle tournament in volume 11. There's absolutely no reason for her to be in these situations other than that she put herself into them to watch the chips fall as they may, and snark on it from the sidelines.
There's the cosplay, too. Not only is it a bizarre hobby/profession, something she's perfectly aware of but in major denial about, but she's set it up--very deliberately and very subconsciously--to be as messed up a situation as it possibly can. There's no reason for her to keep it a secret the way she does, to completely change her appearance outside of the classroom, to narrate her own secret life in her head and cut herself off from everyone else. It's messed up, and that's WHY she does it. To be messed up. To be one of the freaks she claims she wants to avoid.
I think in most of her mind she really does believe she wants to avoid "weird" things at all costs--but it's obvious by the way she's run her own life and the way she insinuates herself into bizarre situations that don't require her presence that she actually feels completely differently. She needs to be around that kind of thing and needs to comment on it no matter who's listening (this habit of making "aside" comments to whoever's standing next to her also shows up a lot in the manga).
So between the insecurity and the TWS, you have her interactions at CFUD. Snarky asides to those she considers "sane," conversations about completely bizarre subjects with completely bizarre people that go on way too long (because she just. Can't. Look. Away), and a lot of hemming and hawing and genuine surprise whenever someone new reaches out a hand to her in friendship or anything more. Deep down she still doesn't really think she deserves that.
So, item: Chisame loves Chizuru, and she loves Mello. In different ways, but she does love them in the "in love" way. And she loves them both because each of them satisfy one of these two impulses she's driven by, and neither of them satisfy both.
Chizuru is like a balm for Chisame's insecurities. Around her, Chisame feels loved, and cared for, and attractive. This is how Chizuru won her over (and, incidentally, how Utena won her over--and entirely different from how Mello won her over). Chizuru's kind, and sweet, and extremely flattering at times, and all in all a sort of rock for Chisame to come back to after the insanities of the day. Chisame feels all light and fuzzy and fluffy and comforted around her. She's someone sane to trade snark with, and dish with, and cuddle and hey yeah make out with!
But it's funny--Chizuru's actually probably one of the most normal people at camp. No powers. No memories of anything strange that happened to her before camp. A teenage girl who knows a lot of pop culture and has a good sense of humor and a good outlook on life, and just happens to be a lesbian and a bit perverted. And Chisame loves her for all that, but at the same time . . . being with a rock is only half of her story.
Mello does nothing for Chisame's insecurities. Nobody feels secure around someone they're desperately attracted to who will never ever return that attraction, heh. And beyond that, he's very pretty, very intelligent, very capable, very twisted, very everything, and Chisame knows she can't keep up (although it does do a lot for her insecurity that he even bothers with her, and that he's complimented her earnestly in the past, but it's not the same).
And then he goes and gets into the most fucked up complicated situations EVER, and the most fucked up topics of conversation EVER, and hey there's that schadenfreude she needs so desperately! There's that taste for the dangerous and bizarre and completely off-the-wall that Chisame continually thrusts herself into the way of. Their very first meeting was fucked up and complicated and weirdly emotional, and went on for entirely too long, because despite the insanity of the zombies and diseases and everything else that CFUD provides--Chisame really needs the sort of bizarreness created by people for themselves to feel satisfied. Mello satisfied her (heh). And then their next meeting was also messed up and strange, and the next, and finally she ends up caring for him and his new boyfriend after they BEAT THE CRAP OUT OF EACH OTHER WITH GUNS and hey, this is exactly what she's been looking for all along! Here's an attractive, witty, charming, mysterious . . . example of everything that's messed up with people in the world. Yeah that's what I'm talking about. A+ baby.
So she has both, the girlfriend and the best-friend-with-regrettably-few-benefits, because she needs them both in her life. Ask her to choose between them, and she couldn't do it, because without her security Chisame's emotionally useless, and without her schadenfreude she's bored as hell and has no outlet for the darker parts of her personality, of which there are many. This doesn't mean she'll be unhappy with Chizuru or that it can never work; she'll be a lot happier with Chizuru than she would with Mello or most everyone else, and she knows this herself, and will try her damndest to make it work. The security she gets there is (marginally, heh) more important than the bizarrities she gets elsewhere. It just means--she'll never be able to tear herself away from him entirely, nor will she be able to tear herself away from anyone else who provides for her the same thing he does, in smaller doses.
END RANDOM BLATHER YAY. ♥