7. Gillian Grayson
Did you ever feel come the start of Mass Effect 2, that Cerberus' involvement and status in the main story was a little too sudden? Sure they were in the first game as a random enemy force seemingly fortifying certain planets and putting up a pretty basic fight - but they were always far from the Illusive Man-commanding super-unit ME2 needed you to believe in. Enter Gillian Grayson, the daughter of a soldier (Paul) who grew up with Cerberus and The Illusive Man from a very young age. Tapped with biotic potential through a series of experiments, she was one of the first humans to ever experience mind-altering biotic powers - along with the books delving into the existence of 'Red Sand'; an on-Mars developed drug that allows non-Biotics to tap into similar sensations. Grayson's revenge-arc takes her across the galaxy from inside Cerberus to even aboard the Migrant Fleet, and she'd be a powerful ally with some great insight to give about the progression of the galaxy itself as humans learnt to cope with biotic powers too.