Suzuka: "Who are you?"
Yui: "I'm you, but stronger"
She's a little of column A, a little of column B. I feel like she's got traits of both of the leads. (she honestly felt like a female Yamato, at first)
Also, based on how things ended at the amusement park, I feel like she's not about to try and steal Yamato. Sure, she sees him as a 'great guy', but she also knows he has a girlfriend, and is very dedicated to her. If she's not a fool, she'll remain friends with both of them and not cause shit (I was hoping for her to be friends with both of them, to be fair, after her appearance). Even if she did try anything, and it actually succeeded, the worry would then be that he's just an easily-swayed guy, which doesn't sound so 'great' to me.
Of course, this assumes the story maintains some semblance of logic moving forward with this situation.