There's a very easy way to do this ICly: Otto wishes to learn self-defense.
Otto decides that he wants to be able to protect himself from the usual nasties that one can find in the cities, or that, being the pimp he is, he needs to know how to fend off challengers or people messing with his women. So he goes and learns basic sword training and refines it from there.
He doesn't even have to be a gladiator to do this. He just has to find a mentor willing to teach him how to wield a tuck and he's set. Then that opens up paths for him to get more serious about it.
Otto decides that he wants to be able to protect himself from the usual nasties that one can find in the cities, or that, being the pimp he is, he needs to know how to fend off challengers or people messing with his women. So he goes and learns basic sword training and refines it from there.
He doesn't even have to be a gladiator to do this. He just has to find a mentor willing to teach him how to wield a tuck and he's set. Then that opens up paths for him to get more serious about it.