A thief living in Ylisse and will do any job if paid the right price. He loves sweets and has candy hidden all over his body.

His support with Maribelle reveals that many years prior to the game, he once tried to ruin her father's good name; he snuck into the royal treasury to frame her father for a crime, which placed him on trial (though he later reveals that his hand was forced with the threat of Maribelle's death and his own imprisonment.) He's been trying to "atone" for what happened since then. In Gaius's support with the Avatar, he bribes them to keep secret about a criminal tattoo imprinted on him after getting caught in a heist, despite their protests and assurances that they would keep quiet. He explains that he does not believe in "something-for-nothing", and refuses to forget about it until the Avatar decides to share a secret of their own. He also is not very fond of bugs, as seen in his conversation with Lon'qu in the Summer Scramble DLC. Gaius has hypoglycemia, which is why he must carry sweets on him at all times.