The Lost Robot
Full Name: Prometheus (a.k.a. R66-Y, Robo)
Symbolic Identities: Lazarus, Prometheus
Robo, the futuristic robot of Chrono Trigger, has parallels in both Greek and Biblical mythology. As his name suggests, he is based on Prometheus, a Greek titan who rebelled against the gods so to bring knowledge to humans. Robo also has a number of striking similarities to Lazarus, the Biblical character who Jesus Christ raised from the dead.
Christ raised Lazarus from the dead after he had passed away from an illness. Similarly, Crono and friends raise Robo from his disfunctional state not only in the Proto Dome in 2300 A.D., but many other times as well. He is resurrected after being destroyed by his fellow R-Series robots, and after he stops working from farming Fiona's desert for so many centuries. He's sort of a super-Lazarus character.
Lazarus was the close brother of Martha of Bethany (Lucca). Notice that Lucca and Robo immediately hit it off and become close friends, which they remain throughout the entire game. It's almost as if they are siblings.
In his later years, Lazarus became a bishop and travelled to a remote region that was plagued with crime, famine, and poverty. There he spent the last thirty years of his life working to develop the region into a peaceful, safe area--which he succeeded in doing. This is just like Robo breaking away from the main group in 600 A.D. to stay with Fiona and Marco to help nurture their monster-infested desert back to a healthy forest. This he succeeds in doing before he becomes disfunctional after so many centuries.
Centuries after the death of Lazarus (his permanent death, that is), Constantine Emperor Leo decided to build a massive cathedral around Lazarus's tomb to honor the work that he did in his life. When the main party travels to 1000 A.D., they find that a massive cathedral has been built around the remains of Robo to honor him. Coincidence? Hmm...
Concerning Robo's element as an indicator of his true identity, Robo has no defined element. However, as Spekkio says, his lasers inflict shadow type damage. Consider what Lazarus is the most famous for--his death. Death, darkness, and shadow certainly have a similar likeness.
Moving on to Robo being Prometheus, Prometheus was a titan who joined the gods during their battle with the other titans, and eventually betrayed the chief of the gods, Zeus, by stealing the Fire of Knowledge from Mt. Olympus and giving it to the humans. Prometheus is known as a symbol of strength in the face of oppression and a champion of humanity for this deed. Well, Robo is similar in that he betrays his God (Mother Brain, parallel to Zeus) and his kind to use their technology (the fire of knowledge) to help his human friends.