Sebastian Marsh

"'Well, well... what do we have here? I don't believe you have an invitation to our little party.'"- Sebastian Marsh to Jack Walters on their first meeting in Y'ha-nthlei

Sebastian Marsh is a businessman, a member of the Marsh family, and the elder brother of Robert Marsh. He is also the Refinery manager (even though his son, Jacob Marsh, actually runs the facility) which produces and hoards the wealth of the Marsh family. As such, he is an essential cog in the machine keeping Innsmouth under the Marsh's control.

Role in the game
Sebastian is referenced numerous times in the game, but is only encountered by Jack inside Y'ha-nthlei, apparently returning from a business trip. He manages to capture Walters with his underlings and throws him into a cell, presumably to make him a new lab rat. After Jack escapes from the cells, he spies on a conversation between Robert and Sebastian. The latter disregards Robert's faith in Hydra's powers and argues they should escape with the research, before his brother reveals he shut down the lab. Accusing his brother of madness, he was told that the Order had no further use for him, and that he could return to the shore. Infuriated, Sebastian drew a gun at him and shot him in the arm, before Robert killed him using witchcraft.

Trivia

 * Despite him being referenced as an older brother, Sebastian's transformation into a Deep One seems considerably slower. If this is due to naturally slower mutation compared to his brother Robert or because he was never treated by the substance created by Esther Marsh remains unknown.