Banri arc finally began lessgoooooo !!!
My guess is that "boyfriend" is the one who talk with Banri at phone with pushy attitude. I mean, at previous ch, she constantly apologizing just because she's a little bit late to pick up the phone. Like heck, that's not even a minute and he already get mad like she's not answering the phone for one week.
And yeah, he looks kinda fishy. With his posture and appearence, how he interact when he met Banri and Arata, etc etc. I wouldn't say he is a stalker and blackmailed her to date with him. But he looks like a possesive and self centered boyfriend.