[quote=MFauli[Wonder why he didn´t use it so far[/quote]

He gets stomach aches. Judging from the he's carrying, he's been using it plenty.

Falco was fine joining teams because he wasn't hung up on beating Izuku. Remember what they said this episode about "Sometimes you have to peg each other down a few notches, and sometimes you have to work with those you don't necessarily like?" He figured there's more to gain by working with Izuku than against him.

I reckon ice guy not using his left hand (flame) is going to be his downfall here.