Saki is way out of his league in my opinion too. She is insecure a bit about not being good enough for him, so she improves herself. She doesn't agree to date him right away, and sees him as more than a handsome guy. When she finds out another girl wants to ask her boyfriend out, she doesn't attack the girl or be hateful towards her.
Saki is the first to initiate something a dating couple should do, go on a date. Instead of letting her insecurities get the best of her and just not use the tickets she was given, she bravely asked hirano to go with her. She's also a girl who wasn't pressured into sex. She let herself decide when she wanted to give herself to him and didn't give into the pressure of him right away. She lets herself decide when she wants to have it, and even though it was only an hour later or so she still made the decision herself.
Can we just acknowledge how much of a butt hirano is? Firstly, he doesn't say that the girl he was talking to was his sister at all. He could've told his girlfriend right away he wanted to keep his relationship quiet from his siblings for a bit. Then when they encountered a girl that was all over him he could've mentioned she was his sister so his girlfriend wouldn't take it the wrong way and then said they weren't dating so it wouldn't hurt his girlfriend at all. Secondly, he has the nerve to lie about not wanting to go to amusement park just to hide the girl was his girlfriend. He could've said, I'm just out with a friend and I'm having a good time with a friend. He had no reason to say such a hurtful thing.
Thirdly, when saki decided to improve her skills as a manager to be more deserving of hirano and started to become more popular, hirano threw a hissy fit. So he's allowed to be fawned over by girls but saki can't dare to be someone others is interested in. Lastly, he didn't stop as soon as she said no to him. It took her a bit to convince him to stop. Believe it or not, if you don't stop touching someone as soon as they say no, you're a scumbag.