It is a bit jumping the gun, but ngl I did too assume that as well with myWelp, I dunno where you guys got that "two guys are time traveling" theory.
The other dude could just be obsessed with Mika all this time, I dunno... the raws doesn't cover anything much of that guy other than "he's been stalking Mika all this time". No indication of whether or not he IS also looping. The only obvious thing in common he has with the MC is the obsessive desire to have Mika's virginity.