Reacter

Description
Reacters will immediately attack before teleporting to the inverse tile whenever you cast a spell. Weapon attacks do not trigger this effect. If the designated tile is unavailable (due to being or due to there being another entity already occupying the tile), they will attack without being able to teleport at all.

Otherwise, it stays passively in one spot.

Strategy
If you want to use a spell to hit Reacters, you need to aim for the tile the Reacter is going to teleport to as the moment you cast the spell, they teleport.

The easiest way to deal with them is to just rely on your weapon. Otherwise, to reduce your mental stack, it's easy to use a projectile spell on the inverse row (as long as you don't cast more than one spell).