Try this, and tell me what you think:
There are several layers in the hierarchy of mutant existence. 3 are known, and 2 are unknown. The first and strongest of the society believes in balance and anonymity between the surface dwellers and mutants. The second believes that they have just as much right to live in the surface as their human counterparts do and hopes to coexist with them or if not possible, enslave them. The third is fundamentally views the different perspectives from a scientific standpoint, and that with all the research done on mutations, they hope to supply enough technology/radiation powers to build a floating city above and away from the surface. These three factions institute their own facilities from sewers, to the insides of secluded mountains, to the point of even having cities underwater and in cavernous tunnels underwater.
There also exist 2 other species unknown to all, even to the other separate unknown species themselves. One race is from the ancient past, who down the line of evolution have mutated and look all too entirely different from what man is today. Whether it was from the Ice Age or even backtracked to the era of the Dinosaurs, one aspect remain true, they have a fully functional cognitive capacity that is in par with any person or in some cases, exceed the normal apititude. They dwell below the earth, building mighty channels across the planet.
The second live amongst them all. They are the shadow, that conceals all knowledge of their existence and have eyes open to all secrets. Some are like parasites living and hiding within the surface inhabitants, some manifest in multiple personality disorders, others do not even know they are due to being well hid in their DNA, and the most pervasive of them all are those who are invisible and hidden in open light or those who can tranverse different dimensions. They have cities in deserts, forests, volcanos, and outer space. They hide when visible, disguising themselves as one of the residents of the places they travel, and live completely camouflaged when seen from the naked eye should they desire to be amongst one of their own. Genetic diffusion helped set their foot on all grounds, and buried a gene most horrific. The one that can transform them, all from decision of their one leader. Activate all their genes at once and results could go from zombification, to mind control, to becoming frenzied and berserk. And worst of all, once a specific gene is activated, it can infect anyone through open wounds. Should their leader ever be killed, the genes lie in wait, and in time, another will arise to take it's place.
Your character is still the lowly thief, and rest is what you make of it.
(Oh, if you run out of stuff to maneuver, you could add in aliens and stuff.)