I think the most "nice/good" thing that could happen is: Either she confesses the situation or at least tries to. Then, bullies show up. He somehow or other notices/realizes. And then it can go 3 ways:
1. Bullies face his wrath or
2. Go the-clueless-transfer-student-is-assertive kind of route - MMC reminds of him a bit minus confidence
3. Both. With his friends(they seem like powerful allies) going 1 while him 2.
Also, Imho his sister will be nice. Might be hostile at first because of obvious reasons. Sadist personality is one thing and being nice is other. Also, whatever info we got of her, she seems to be like that just to that guy(I forgot the name) who likes her.
All these are just my guesses/opinions.