BioEditBoss; "Three-eight", or "38" (RC-1138), was a Republic Commando who was assigned to Delta Squad; a team of elite of specially-trained Clone Troopers; and served as their leader under the rank of Sergeant.
Boss is/was one of the more experienced Commandos, who was often quiet and stealthy while leading missions. He even personally knew Jango Fett before he became corrupt and greedy.
Interestingly, Boss managed to learn Jango Fett's Concord Dawn accent and speech patterns, despite his upbringing on Kamino (being cloned from a Mandalorian). Like his fellow squadmates, Boss participated in the search for Ko Sai (wanted enemy), and did whatever no average Jedi or Clone Trooper could do like the other Republic Commandos.
Boss wears the standard Commando armor like with every other Republic Commando, only with unique orange markings and wears a standard datapack; like with how every Commando wore before they each had the option to distinguish themselves from one another.
Boss' code-name or number is loosely based from THX-1138, one of the very first films produced by George Lucas, and the initals "THX" and "1138" have appeared in many Lucasfilm-related productions and trademarks. (THX Sound.)
He is also one of the most well-known Republic Commandos.
Star Wars: Republic Commando Videogame (First Appearance)
Star Wars: Republic Commando Novel Series
Star Wars: The Clone Wars "Witches of the Mist"