Little is known about Juan, but he must work for Stephen in some capacity.
The first time America got a good look at Juan, he was spraying water from a hose onto Dr. Colbert, who has just come in second place to South Korean pop sensation, Rain, in a late-night dance/cuddle off.
Stephen just knew Juan was doing it on purposeEpisode #391.
Perhaps Juan is the studio gardener, or perhaps he is the building maintenance man working under Tad? Either way, someone has to take that hose away from him.