Fixer (RC-1140) was a Republic Commando who was a member of Delta Squad, and served as their computer specialist.
Fixer was second-in-command of Delta Squad, and was noted for his use of callsigns (CC numbers) instead of the nicknames the clones used. His nickname came or derived from his love of technology, and his ability to slice into computer systems.
He also did whatever an average Jedi or Clone Trooper couldn't do, like with the rest of his team members, as well as the rest of the Commandos.
Fixer has the standard Commando armor like with the other Republic Commandos, only with unique green and in some cases purple markings just on his helmet and having his own customized datapack consisting of a scanner for computer systems.
Fixer very much resembles how real-life US Army and other military soldiers refer to each other as by their callsigns. Such as their lastnames, and by their I.D tags or "dogtags".
Star Wars: Republic Commando (videogame; First Appearance)
Star Wars: Republic Commando Novel Series
Star Wars: The Clone Wars "Witches of the Mist"