Sentomaru is the bodyguard of Vegapunk and is in command of the Marines’ Science Unit as its captain.
During the Egghead Arc, Sentomaru decided to ally with the Straw Hat Pirates out of loyalty for Vegapunk, leading to his acknowledgement as a rebel by the Marines.
In the recent SBS Vol. 106 Oda revealed Sentomaru’s backstory:
The marines received a report saying that wild bears are attacking human villages on a certain island, and Vegapunk accompanied them to set up a system to fight bears. When the marines were investigating the dangerous mountain, they came across a boy who defeated all the bears with sumo and reign as the boss of the mountain, and that was Sentomaru. He was a boy who was abandoned by villagers due to his violent behaviors when he was younger. Intrigued by the strength of this boy who didn’t return to the village and is always hungry, Vegapunk hired him as his bodyguard.
