If it's going to be a heavy roleplaying game that will be centred in cities a lot, then I can recommend a changeling assassin, focusing on first strike novas. The assassin is a very weak striker, but in a heavy roleplaying game, especially one that tends to spend a lot of time in cities, it's freaking awesome.
Give him a weapon that can change into any other weapon at-will, and armour that can change into any other armour/clothing at-will, and he becomes a nigh-unstoppable infiltrator. Teleport at-will and being able to walk through walls (sustain minor and an encounter power, so only requires 5 min reboot), and being able to impersonate anyone (with an extra +5 bonus on an already high bluff check) wherever you go = loads of fun.
But if those two criteria aren't met, they can be a bit lacklustre, with their awesomely cool abilities going to waste more often than not, and their weak striker capacity making them a poor choice next to the rogue.
Personally, I'd probably make a drow rogue as her companion. A drow rogue and a tiefling warlock would make a great 'dark' partnership. He could be her 'defender' in a sense, being focused on melee and using the drow longknife and the heavy blade feat (can't remember the name, allows a rogue to use heavy blades for sneak attack and powers), whilst she darts in and out. Both classes have a pretty strong secondary 'controller' schtick so the two of you could manipulate the battlefield like nobody's business
