Dex

Killing enemies, completing quests and using datachips all grant you exp toward levels in Dex. I highly recommend upgrading your lockpicking to the max as soon as you can in order to take advantage of the ample number of lockers and buildings to pick into granting you a ton of experience. By the time you get back from the Unknown Location, if you have done the majority of sidequests prior to it, you should be hitting level 15.