Without a shadow of a doubt it has to be Val, that bloody woman in FarCry. After getting herself captured, making you spend the next god knows how many hours crawling around on your stomach so you don't get turned into a colander by the tiny little dots with guns on the horizon, she then throws sarcastic insults at you and decides to join you in battle.
Now for the next three or four missions the carefully honed ability of distracting soldiers and taking them out silently with the MP5, the only useful weapon in the game (unless you have to fight a trigen), is absolutely sodding useless as she won't find cover. Just when you think that that is bad game design, you suddenly realise the enemy can't even see her. Then to make things even more preposterous if an enemy gets close enough to take out with the MP5 she start firing at him, and suddenly he can see her.
It's moments like this that you really empathise with the AVGN on one of his many tirades.