Here we have a clue as to why Tommy beat the MC in EP6. Josy talks a lot about the MC. It doesn't matter if Josy personally talked to Tommy about the MC in EP6, because her father Pete knows about the MC, Tommy can know it from Pete. In EP9 we find out so Tommy wants to protect.
Check out the 3 screenshot! The MC is holding Josy's eye where the MC got the black eye from Tommy. I can therefore imagine vividly, so that Tommy in EP10 reacts a little angry at the beginning, but quickly calms down again, because he has known for a long time about the crush (MC) of his sister Josy.