Nemo

Rider is a Phantom created from combining a Heroic Spirit, Nemo, with a Divine Spirit, Triton, due to Sion lacking a proper summoning foundation like FATE of Chaldea.

The Nemo Series is an ability inherited from his Master's Memory Partition ability, allowing Nemo to divide up his skills into alternate selves to properly manage his ship.

Nemo was an Indian prince who went into self-imposed exile aboard the submarine he made himself, the Nautilus. Triton was a Greek God of the Sea and a son of Poseidon.