Bio

The background of Boss is set by Star Wars canon. This is the blurb from Wookieepedia:

RC-01/138—simplified as RC-1138 and also known as Delta-38 or by his nickname of Boss—was a clone of Mandalorian bounty hunter Jango Fett. Born in 32 BBY on the watery world of Kamino, Boss was trained by the Mandalorian Cuy’val Dar training sergeant Walon Vau to become a clone commando in the Grand Army of the Republic. Placed in command of three other commandos, Boss lead the elite special forces unit Delta Squad as their sergeant. When the Clone Wars broke out between the Galactic Republic and the Confederacy of Independent Systems, Boss took his place in the Grand Army’s Special Operations Brigade under Jedi General Arligan Zey, and served alongside his clone brothers in Delta Squad on a number of battlefields, ranging from the deserts of Geonosis, the halls of the RAS Prosecutor, and the streets of Coruscant, to snowy Mygeeto and the the plains of Haurgab. Following the transformation of the Republic into the Galactic Empire, Boss and the other members of Delta Squad—minus squadmate RC-1207, who the squad was forced to abandon on Kashyyyk in the final days of the Clone Wars—served in the new Imperial Army, as members of the Imperial Commando Special Unit, a specialized unit within the 501st Legion tasked with hunting down clone deserters, fugitive Jedi Knights and Jedi sympathizers.

—-Wookiepedia, RC-1138

That much won’t change.

Here are some basic physical and mental traits.

• He is 1.83 meters tall or 6’ even and built. Not overly huge muscles but because of his Commando status, the training caused his body to grow larger than others
• He has black hair and brown eyes
• He has a faint 3.5” scar from the bridge of his nose to his left cheek from a sparring accident gone wrong during training
• His voice is eerily similar to Jango Fett’s voice (his template)
• His accent is Concord Dawn-like (think New Zealand)
• If you gain his trust, he’ll do whatever he needs to to protect you
• He’ll reminisce about the good times in the Grand Army of the Republic
• Drinking is a social sport for him. He’s not one to do it alone or without reason. Mourning by drinking, however, is something he does not do
• Relationships are something Boss is unfamiliar with. He’s heard of Darman and Etain being married but he had no downtime of his own. 
• To go further with relationships, he’s really seeking companionship and love wherever he can get it. Male or female. Love knows no bounds and the Mandalorian society recognizes that

His non-canon timeline.

• The current estimated date in the Star Wars Galaxy is 17.5BBY (easy for most SW people to recognize)
• After having to abandon Sev, he and his squad worked for the Empire for a few months hunting down Jedi. They later abandoned the Empire and sought refuge on Mandalore with Walon Vau.
• Around a year after Order 66 was issued (18BBY), Boss, Scorch, Fixer and Walon went and brought Sev back from the Trandoshans. 
• This post deals with how Boss got to the Milky Way Galaxy.

Any questions? Shoot me a message. And make sure to read headcanons.