She used to work for the Faction Paradox...well actually some offshoot low-rent Faction members called the Remote. Basically, the Remote use media signals to control people...like the way a television remote controls TV OH SO META MILESSSSSSSS
Anyway, the Remote are beaten by a teenager and then Compassion joins with the Doctor because... for fun?
Okay, I'll stop that now. Anyway, Compassion is noted for being cold, clinical and not compassionate (bravo miles), which contrasted her greatly with both the last companion, who was so moralizing she'd let a dolphin rip her entrails if she thought it would save the environment, and to the Doctor, who often acted the way people think ADD kids act. Instead, Compassion couldn't care less about your puny romantic emotions. She thought like a machine, in a way.
So of course, in their infinite wisdom, the EDA writers turned her into... ah, that would be spoilers, wouldn't it?