What to do with Leif ?

S Drink is a Leif-exclusive skill, so it can't be foddered, and Drive Atk is not worth foddering a 5* exclusive unit (it's really not even that great in my opinion).

I'd merge.

What are you gonna fodder? Steady Blow? I'd merge them...

merge, that's what I would do too. not worth foddering off

Merge him, he is a good unit, with s drink he can proc 3 turn specials like Luna, Draconic Aura or Bonfire in the first fight if he can double.

Unless you have a unit that has a burning desire for steady blow or drive atk, I’d say merge him. He’s still a great unit to use in TT+.

I'm obviously biased because Leif is one of my favorite characters in FEH, but like everyone else has said, I'd recommend merging him. He has a fairly high BST, and makes good use out of all of his stats. He's also super flexible and can be built around Light Brand (probably getting a refinement eventually) or Slaying Edge / Wo Dao.

Mine is +SPD/-RES and this is the build that I run on Arena Defense. For PvE I normally run Moonbow and Desperation instead, and he's probably one of the more reliable sword units I have.