He is an alien outlaw, obsessed with owning the latest technology. However, his plans are often poorly executed, and he is frequently captured by Miles Callisto and his family.
Also despite the fact that he is the main antagonist of the series, he is not dangerous to others as he is solely interested in technology. He is better described as an annoying and persistent pest, or as Captain Joe Callisto called him, "the biggest pest in the cosmos."
Gadfly has little technology of his own, other than a rusty vessel and two holographic goons. He very much wants to have a jet pack. Gadfly enjoys annoying Miles by saying his name wrong. This changes in "Unexpected Ally", when, after saving Miles' life, he drops the habit out of courtesy, and to follow Cosmic Explorer Rule #5: 'be nice to everyone'.
As the antagonist, Gadfly serves to be the counter to Uncle Joe for Miles. While Uncle Joe is who Miles aspires to be, Gadfly is a warning to Miles of who he may be if he does not follow rules. After all, rules are what Gadfly hates. Gadfly gets together with other space thieves, forming a group, though they don't work together to steal things due to their greed.