I like the character of Yui, too, but I disagree that "most girlfriends would be a bitch". Maybe that's a popular yaoi trope, but I think it's a sloppy reflection of life. I like how Kinoshita Keiko portrays Yui because she's realistic and mature for a change. Yui is what most mature women (and men) are like; unwilling to put up with a suitor who doesn't love them, and gracious about letting him (or her) go.
Yeah, I was kind of being prejudice, but I have read quite a few yaois where the ex will try to break them apart. Then again I'm sure there are probably an equal amount where the ex is either nice about breaking up or even supporting the couple.
I like Yui a lot, because a lot of girlfriends will usually be a bitch and try to sabotage the relationship, but Yui understood and even encouraged him. I like that :) <3