Sentomaru is the bodyguard of Vegapunk and is in command of the Marines' Science Unit. After the timeskip, he becomes a full-fledged Marine officer.
Sentomaru is a large man with a stocky build similar to that of a sumo wrestler. His face and haircut, resembles a look of a woman, except for the large scar extending from his left eye to the left corner of his mouth. His outfit consists of a shirt that resembles a giant brown bib or apron with an emblem meaning "heaven" on the front, and a large red and white rope or "tsuna". Sentomaru also has bandages wrapped around his right fist and elbows, along with the Kanji Sen tattooed on his left shoulder. Sentomaru carries around a huge, two-edged axe, as tall as himself, and just as large.
Following the timeskip, he is shown wearing a Marine coat over his shoulders due to his acceptance into the Marines. He has also removed the bandages around his arms.
You can download this cube papercraft model from here: The One Piece - Sentomaru Cube Craft Free Paper Toy Download