Sans

The War Against Dark Matter
Sans was the one who sealed pieces of Dark Matter away on Earth 1. However, Dark Matter sent some of his pieces to the Pokemon world, so Sans had to turn The Controller of the Culdee Fell Railway into a Pokemon to stop him.

Role in the 14xx series
Sans put a piece of his power inside of Diesel 10 for unknown reasons.

He was tricked into working for Butch by being told Sodor was taken over by workers of Dark Matter. However by the time he realized his mistake, he had already fused with the glue. This glue corrupted his mind and turned him into the monster, Kirby. This was not the real Kirby, but rather a monster that took his form. Kirby was taken out by the Soviet League. This left Sans badly Injured. He was bleeding out. He told the Soviet League to kill Butch, and him. Dave shot him to death and he started burning, before Dave shot him again