As far as I can remember about this manga, I think it was pretty much a misunderstanding due to not being open with each other/lack of communication and due to the guy's complacency resulting to the girl to act and think like a bitch.
The guy doesn't want to distance himself from his best friend...
I'd say Ruan Mei if you want superbreak since she's the best at it and good in DoT and dual dps setup.
Superbreak - March 7th/Himeko, Ruan Mei, HMC, sustain
DoT - Kafka, Ruan Mei, Sampo/Guinaifen, sustain
But if you prefer FuA, Robin. She also has the edge in DoT and dual dps setup compared to...