Theory 1:
http://puu.sh/8wsPo.jpg[]
Theory 2:
Riki is obviously very feminine and meek. He can effortlessly approach girls and talk to them completely naturally. Girls instantly drop their guard around him (heck, most of them don’t even think about putting it up in the first place) because he’s giving off that “absolutely harmless” aura. Their next thought is “He’s kinda cute”.
So then, when he pretty much starts off at a range where he can fire off point-blank shots, any little triviality will do.
Spending enough time with the girl to make his presence as natural as air, a cool line, a bright smile or even something as simple as picking up her eraser is enough to flip the romance switch. Remember how it was with Kud? She got a crush on him pretty much on the same day they met because he was the only one to not laugh at her name.
I bet it was something similarly simple in Mutsumi’s case.
The motherly feelings thing sounds plausible when the target is a ‘mature’ type.
