The powerful and mysterious Proxy appearing before Re-l at the beginning of the series, subsequently thrashing Re-l's apartment in his fight against Monad.
Ergo appears many times throughout the story, and eventually, the secret of his identity is revealed...
Trivia: Character's name is a combinanion of two latin words - "ergo", which means "therefore"; it's source is famous Descartes statement "cogito ergo sum" - "I think, therefore I am". Cogito is also a name of an virus attacing autoreivs. "Proxy" is probably a word taken from phrase "genus proximum" - "superior spieces". It referes to almost-godly Proxie's nature.