Twilight was my first pvp tournament and my MVP was Gardevoir. With confusion, it would annihilate Toxicroak faster than Venomoth while also countering all of Toxicroak's poison counters.

With Mirror Cup allowing us to replay Twilight, I've been revisiting Gardevoir which has a killer new fast move, Charm. What I've discovered is that the two Gardevoirs gave mirror match-ups (sorry the pun was irresistible).

Confusion Gardevoir hits poison types hard (except Tentacruel) and loses hard to dark or ghost types. It also loses to other fairies. Its a high risk, high reward pokemon - you win hard or lose hard.

Charm Gardevoir loses to poison types (except Toxicroak) and wins hard against dark types. Its got a mixed record with ghosts, beating sableye and losing to froslass and haunter. It beats or ties other fairies, though it loses to Azu.

So if you bring a Gardevoir, you can keep your opponent guessing about what to bring against it. Because Gardevoir hits so hard against its prey, that extra time for your opponent to see what moves it has and switch and turn the tide in a close game. Its especially effective if you send it in against Toxicroak - they won't know what to send in against it. It can also make their trio selection a little more difficult.

While there are arguably better fairies than charm Gardevoir (Wigglytuff beats ghosts and has more solid wins against fairies) and arguably better counters to Toxicroak and Tox counters (Sableye also beats tox and all non-fairy tox counters), Gardevoir performs solidly in both roles and keeps your opponent guessing. If there wasnt a duplicate restrction, I would bring 2 Gardevoir.

Edit: Added the point re: Toxicroak. Send in a dark/poison as your lead, if they lead Tox then swap in Gard. They wont know if its charm or confusion and wont know who to swap against you. By the time they figure it out, Tox is half dead.