Rennes - aka "The Wren" - is a character from the first Cube film. The character is portrayed by Wayne Robson and is the second person to die in the film.
The Wren is a convict and master escape artist who had previously managed to escape from seven different prisons. Likely the organisation that built the Cube wanted to see how well a man of his talent and reputation would last within their construct. One can only wonder how they reacted when he was the first to die after all the test subjects (excluding Alderson) united.
Rennes triggered one of the Cube's many traps and wound up having his face hosed with acid. The acid quickly ate away the front half of his head.