-Def +Atk Bike build

If you're not willing to wait for another Bike and not fodder the only one you have for Steady Breath, -Def isn't going to make him useless. At least you get more damage potential, which will allow more healing when you swing that big Aether so any extra damage you have taken from the lowered defense can be neutralized. Steady Breath also patches this up somewhat. The worst that can happen is a +Res with a -Atk or -Def.

My first Bike that I used up until this recent banner was +RES -DEF and he was still a great unit with the default. Your boy will be fine; don't forget that A skill is giving him a boost to that DEF anyways. If you can I would suggest giving him one of the DEF seals (def+3, distant def, etc) against tougher enemies however.