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, placing people in stasis, 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.
It is often speculated that he might be a member of the Q Continuum, but little evidence exists to make a proper connection. His parents looked and acted like the Organians did, further muddying the waters.
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.
His parents seemed content to let him amuse himself at the expense of the crew of the Enterprise until he seemed to be on the verge of causing permanent harm, at which point they intervened.