Sets Trick Room and benefits from it — Hatterene is slow. Magic Bounce reflects status moves and hazards back at the user. Enormous Special Attack with Psychic and Fairy STAB. The most reliable backup Trick Room setter.
Why it's threatening
Magic Bounce automatically deflects Taunt, sleep moves, and other status attempts — common counterplay against Trick Room setters doesn't work on Hatterene. You can't Taunt it; you can only hit it or redirect around it.
Dark moves deal 2× and bypass Magic Bounce entirely. Poison and Steel moves also hit 2×. Fast Dark attackers can KO before Trick Room is set. Fake Out is reflected by Magic Bounce — use direct attacks.