Neither Erio nor Caro get any screen time time after the train incident, so it's hard to say whether he really is that useless or not. But if you look at the shots where he is with the others, you see how small he and Caro are. Maybe they mentioned how old he and Caro are, maybe not, but he's gotta be really young to be so tiny. So, I guess it's reasonable he's not showing huge prowess in battle yet. Still, let's hope he will be in a real life or death situation sooner or later and will then show the girls who's the Man...