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. Sebastian has been loyal to Halldor's family from the time he was a young boy, but allowed Halldor to travel the world and mature on his own, realizing this is a journey for Halldor alone. Sebastian rarely displayed emotions other than shedding tears from various emotions. He shed tears of joy after seeing Halldor became worthy of Odin and shed tears (of sadness or joy) from Halldor's defeat by Yusei's hands.
In episode 150, he is cheering on Yusei.
His name may be a reference to the anime and manga 'Black Butler' by Yana Toboso.