My question regards the set effects that require all 3 units and the weapon to complete the set.

Is the 4 piece effect only active while swapped to the weapon or does having it equipped activate it?

I see the full Luther set + weapon gives +120 Melee, +120 Range, +120 Tech, and + 10 PP, but if it is only active while attacking with that weapon, it isn't really worth it.

It must be your active weapon for the bonus to apply.

@AndrlCh sadness has overcome me.

Easily worst potential in game lol

Honestly, cases like this are probably the reason why they mostly got rid of Set Bonuses with 12☆ Units.

@AndrlCh very likely. The 3 piece still gives +60 to all and 5 PP, but I will just stick to Brissa (unless I can get the 4 set for Tagi for my Katana class).