Which hero promote?

Depends on your team really. I really like Mae but the only reason why I use her is because I got a +Atk -Def and gave her Attack +3 skill with a +1 merge to get 53 total attack. +Spd isn't going to do much, her speed is pretty bad. Camus is the best option I would say just because he can be a lot more versatile than Zephiel

if you don't have a blademage yet, promote Mae. 36/34 offense stat is pretty good. If you already have one, get Camus. He is a really great unit.

Mae +spd is fine if you still have no other good tome user. Even with Owl you will get atlleast one ally near to you 36spd and block with that most double.

Camus is a great Hero too thx to distant counter build and well rounded stats he can be put in your team even without Horse Emblem but would put on him then QR that he can still be used as lure Hero. And not to forget he will be the next Arena Bonus Hero if you choose not to pull from next Banner.