Thieves and rogues of all sorts of backgrounds populate Karameikos. There are street urchins, assassins, spies, slavers, rogues and more. Some are part of a thieves' guild, but most go it freelance (however, they find it difficult to get their own fences, find anyone to trust and elude investigators).
For those wishing to be a part of a guild, there are three thieves' guilds from which to choose.
The most popular guild for thieves and rogues is the Kingdom of Thieves, which provides fences, help when needed and even occasion jobs-for-hire. The Kingdom of Thieves is ruled by the mysterious Flameflicker, the thief king whose face is not known, and whose many crimes are legendary.
The Veiled Society is another that operates with mafia-like organization. This guild is perfectly suited to thieves who like hurting people as much as they like robbing them. Its main center of activity is that of Specularum.
Thirdly is the Iron Ring, a secret organization that engages in slave trading in Karameikos. The most cruel and evil of the three guilds, its members include those of all classes, whether mage, thief, fighter or evil cleric. The Ring operates throughout Karameikos, and only a select few know the identity of its master(s).
Congratulations, you have now successfully created a Mystaran character!
For further development of this character, consult your Dungeon Master.