Byleth vs Bowser

Just juggle him. If Bowser is above you, you win. You can shield his down air and punish it, and roll or spot dodge his down b and punish that even harder.

Bowser will really struggle if you space well, you can especially punish Bowser's landings well and they really can't do anything to you from the air. Just watch for out for their side-b.

Good matchup, but if he gets too close, you will lose. Use fair and neutral b to keep a good distance and nair if he gets in your face. Only use side b if he's in the air.