Inherit Blade Tome to Leo or keep Tharja? (Or Unlock Potential something else?)

I pulled a Leo on my frer summon from th refined weapons banner a little while ago. I'm pretty sure it's +RES/-SPD. I'm unsure how to build him and I kept seeng the Blade Tome suggestion everywhere, so I was going to just go ahead and Unlock Potential for a random Tharja I had laying around in my barracks. And when I was going to finally unlock her potential, it tured out she was +ATK/-HP, which is apparently really good! So now I'm stuck and unsure what to do.

Leo would have a Cavalry team back him up with buffs and an Erika with a refined weapon (+EFF) for Tharja (if not Legendary Ike).

Help would be appriciated!