Mason is a one-off trainer who shows up in "Outrageous Fortunes" (Episode 249) to challenge Delaney. He's actually unnamed in the original Japanese version.
Mason is the seventh trainer in a row to face off against Delaney, and he's got a specific goal in mind: he wants his Hitmontop to finally break Poliwrath's winning streak. It doesn't go his way, though. Delaney's too strong, and Mason's Fighting-type gets shut down pretty quickly.
The character's name comes from his English voice actor, Mason Ripka, who was one of several kids who appeared in the show as an extra.
Content compiled by AnimeList.moe from publicly available sources.
