Marlon's Jellicent is Marlon's most powerful Pokémon in the games, which also makes an appearance in the anime as well.
Marlon's Jellicent was only seen shortly in battle, where he battled Cameron's Ferrothorn. Jellicent dodged a Pin Missile attack from his foe, he used Shadow Ball on Ferrothorn, but Ferrothorn dodged. The battle was decided when Cameron ordered Thunderbolt on Jellicent, which resulted in a in a knockout. Ferrothorn would've been able to beat his whole team, but before Jellicent fainted, his Cursed Body Ability disabled Thunderbolt, making him defenseless against Marlon's next Pokémon Mantine.