Ghost Ninja's use it, but it doesn't make everyone target that character like it makes it sound, it just adds a small dmg buff to the focused one. Not very worth it atm imo.
"Focus fire increases damage to the marked character and combos off certain character's skills. It does not cause characters to focus their attacks on the marked enemy"
Demon King was my first UR too and she's been sitting benched, just don't have a place for her. Maybe now with access to cannons but I want to go Temple.