Marcellus is a trainer from Scarlet City on Red Rock Isle who appeared in "Octillery the Outcast." He's best known for the emotional arc involving his newly-evolved Octillery and its struggle to fit in with his tank of Remoraid.
Just before the Whirl Cup kicked off, Marcellus challenged Ash's group to a battle—specifically hoping his Remoraid would evolve into Octillery and give him a competitive edge. Misty took the bait with her Corsola and lost, which was exactly what Marcellus needed. His Remoraid evolved right then and there.
But things went downhill fast. After bombing the preliminary rounds, he came clean: his other Remoraid had rejected the newly-evolved Octillery so harshly that neither could focus during their battles. The whole situation had him pretty down.
Then Team Rocket showed up with a vacuum truck and stole his entire collection of Remoraid, because of course they did. Octillery managed to leave an ink trail leading back to them, and Marcellus and the group tracked them to a port. They got the Remoraid back and saved Octillery from a nasty fall—and something about the rescue clicked. All the other Remoraid suddenly evolved into Octillery too, and they finally accepted their old friend back into the group.
By the time we last see him, Marcellus is watching the Whirl Cup arena drain its water alongside Ash, Misty, Brock, and the other trainers who'd made it to the tournament proper.
Content compiled by AnimeList.moe from publicly available sources.
