I personally understand that she was bullying him, then she got bullied herself later ; so MC is being nice to her for his revenge, not that HE was the one who started the bullying back. Otherwise why would he play the nice guy for revenge, if he already succeeded at planning her bullying back.The mystery is intentional given that we have no internal thoughts from MC. It's stated she used to bully mc. Seems pretty clear the author is intending us to believe her being bullied is orchestrated by MC and you gotta keep reading to find out if MC is torturing her or actually just innocently being nice.