Saul is a giant, but not an Elbaf giant, being born elsewhere other than Elbaf. At some point in his life, he became a Marine and eventually made it to Vice Admiral. Before the storyline, Saul had experienced travel in the Grand Line, though nothing was said of his adventures other than the fact that he met many Devil Fruit users.
Saul takes the role of the big friendly giant who is somewhat shy and timid. While he was loyal to the Marines he questioned his orders at least twice, showing he followed 'Moral Justice' over their code of 'Absolute Justice'. Saul is also characterized as having a unique, but awkward laugh ("Dereshishishishi") that he finds somewhat embarrassing. He enjoys seeing others laughing and having fun and encouraged Robin to laugh.
He was one of the three people who influenced Nico Robin, Saul wears a cowboy hat which Robin would wear herself. He encouraged Robin to laugh even if it hurts which explains why she smiles at the time of trouble. Because of his middle name, this explains how Robin knows about the Will of D, she even told Luffy later why would he fight, the will of D.
Saul and Vice Admiral Kuzan (later known as Admiral Aokiji) also had a good friendship, and they both shared the same sight on Moral Justice. The reason Kuzan allowed Robin to live was mostly a final favor to Saul.