Trelane appeared to be human, but this was apparently just a choice; his parents manifested as energy clouds.
Trelane could alter reality at will, causing matter to appear or teleport from an unknown location, teleporting crewmembers from the Enterprise, and seemingly causing a planet to continuously reappear in the path of the Enterprise, despite their attempts to avoid it. At least some of his feats apparently relied on some kind of technology kept on the planet.
Trelane was apparently a juvenile of his species, and his immaturity was evident in his behavior. His sole motivation appeared to be his own entertainment.