The ending was sudden, but I think people are overstating things being unexplained. Most of their interactions have you wondering whether she's being honest or just teasing him, because she DOES tease him frequently for fun, but the ending should make it obvious that she was just being honest about her feelings for him most of the time. Azusa liked him from pretty early on because of how he treated her, but Yuuta's self confidence issues made it hard for him to interpret her behavior that way, understandably. If you reread a few chapters from earlier I think you'll understand her character.
Thats kinda the thing, her character is interesting but we dont know her that much at all.
Lets consider for example, how quite often she -who was always so flirty- seemed to simply dissapear when MC tried to talk her through phone; leaving him waiting long periods of time until finally a date was agreed to meet in person. It's like, she was forcing him to be more assertive and pursue her. Which I guess was kinda her intent, to make him be more self-confident about his chances with her, and that cool, but it makes me analyze this whole meeting in the final chapter...
Because if Mc hadnt asked for the meeting, she wouldnt have either.
By her own comment, she was sorry for coming on too strong with MC during their previous date, but she never apologized when that happened. At the time, she just acted as if it was okay, that they didnt need to have sex if he wasnt comfortable, and the next day she left without a word.
She was interested on him, yes, but also she was totally okay with just leaving and never coming back.
In general, she always behaved in that kind of sus way, being very flirty and all, but also very distant and guarded. As if ready for being dumped or dumping the other person.
Again, lets remember her weird comment during the previous date about her enjoying to take guys' first times, what the fuck whats up with that...? Maybe she had dated a lot of introverted like MC , but they just thought on her as a slut and dumped her after having sex with her once, and thats why she made that sort of comment? But again, what would she gain from dating inexperienced introverted virgin guys like MC? Unless, she truly just liked each one of them -just like MC- for real. Maybe nerds are her type?
Anyway, it makes you wonder what kind of (possibly bad) experiences she could have had with previous lovers to make her behave in such a way.
...But we never got her backstory so we'll never know.