Yu-Gi-Oh! 5D's Episode 123: "Eyes on the Prize"
Sebastian is Halldor's butler and a part of Team Ragnarok's pit crew. Sebastian has been in Halldor's family's service for an apparently considerable amount of time as he knew a great deal about Halldor's mission involving the Aesir.
In episode 150, he is cheering on Yusei.
His name may be a reference to the anime and manga 'Black Butler' by Yana Toboso.