A 70-foot-tall intergalactic diplomat from another civilization. Telseth claims to be a representative of "the star-spanning Kvurri," interested in peaceful contact.
In 2013, a giant door mysteriously materialized over the Gaines River one day. When it opened, it was the being humanity would soon come to know as Telseth who stepped through, announcing his diplomatic intent and requesting a human guide to Earth's cultures and customs. Ben Pullam volunteered for the position. ("Through Open Doors (Part One)")
The door stayed on the Gaines, a physics-defying portal to his base... somewhere else. Eventually, the mobster Thatcher Jerome, who was in charge of the river, tried to shake down Telseth, who was after all in his territory. For his own reasons, the Ambassador chose to enter into an arrangement where Jerome's men would deliver to him whatever Earth goods he required. Over the course of these deliveries, he came to know Jerome rather well. During one delivery, however, the human stole an alien object he saw lying around. The Ambassador seemed oblivious, though who can really say? ("Through Open Doors (Part Two)")
The extent and specifics of his power are unknown, but they're presumably vast indeed.
The doors in the sky which he used to arrive on Earth were unaffected by the combined powers of Samaritan, Winged Victory, N-Forcer, Reflex 6, American Chibi, and E.A.G.L.E., suggesting technological capabilities which are considerably beyond current Terran science.
He was walking on air when he first stepped through those doors, and he telekinetically enabled Ben Pullam to do the same when they retreated back through together.