The idea I had was to make him an average civilian-turned-rogue. He began his career as a watch/locksmith, but was at some point forced into to a life of roguery due to unfortunate circumstances outside of his control. Rather than being a standard thief, though, I imagined a sort of sleuth or private detective, maybe a fantasy version of Sam Spade or Phillip Marlowe, but less mercenary. Maybe Jim Rockford in the Rockford Files is a good comparison: a truck driver who decided to become a gumshoe, and who thinks that he's hardboiled, but everyone else knows that he's just a big softy good guy (and he's eternally broke because of it, since he's always doing favours for victims who can't afford to pay anything). As such, the skills he has focus first on tinkering with locks and traps, second on information gathering and dealing with the underworld, and thirdly with actually 'pulling a job’.
Here's a vague idea of the background I was considering: Mixnan, who had started out as a law-abiding and respectable locksmith, had found that the nature of his trade put him in contact with some pretty nefarious characters. Just by going about his business he learned all sorts of ins and outs of the underworld; who was who, how one went about doing this or that. Often a shady individual would enter his workshop and try to convince him to disable or replace the traps they 'forgot' they had installed on a chest. On more than one occasion he had been sent to some aristocrat's recently burgled manse to replace the treasure vault's locks, only to discover from the way the traps were deactivated that the burglary had to have been a set up, cleverly staged to give the public impression that the lord's prized magic item (which in turn had been stolen from the thieves' guild) was no longer in his possession. At any rate, after digging his nose where it didn't belong one too many times (or two), Mixnan found himself on the wrong side of too many criminals, and had to hide away until the assassins stopped coming for him.
Now, years into his life of adventuring and mixing with the underworld, Mixnan has found that he simply has a knack for finding trouble, and the people who mean to cause it. He's a pretty good judge of character, and can generally eyeball someone for a sucker or a blackmailer, but for all that he doesn't know himself very well: while he likes to think of himself as a self-interested party, only willing to snoop around or build a trap if the pay is good enough and the risk minimal, those who have sought him out can tell that he hasn't got the stomach to take advantage of an innocent person, or the heart to turn down someone in trouble.