Byleth has impressive abilities and can fight at just about any range but Sonic takes this one with ease. He’s just way too fast for Byleth so she just won’t be able to land any decisive hits. Sonic has a lot of raw power of his own too so a few good shots will end the match. There just isn’t much that Byleth can do at that point. Sonic wins.
Suggested by iKnowledge Guy is a capable fighter with a lot of very fast hitting combos but he isn’t quite ready for a true warrior like Byleth who has dragon abilities to amp up his usual stats. Guy will be on the defensive from the jump here and has no answer for the various projectiles and other special abilities that Byleth will be shooting his way. There’s just no escape from them. Byleth wins.