good users of atk/def bond?

Masked marth
Lucina
Chrom
Smolzura
Close counter units
almost all Armored units
Flora (depending on build)
Fjorm
Owain
Sanaki
Shiro
Lukas
Sharena
Beruka

Almost everyone in the game can use it with the corrects build.

Armor and dragons. Like el bastardo said, you can use on almost any solid character (won’t recommend super weak defense mages unless you build them to take a defensive hit)

Im using the seal with Anna in Arena and it works very well because she lacks both atk and def.