Can a co-signer take your car away?

It depends on how the vehicle is titled. If both of your names are on the title she has an equal right to possession of the vehicle as you do. So technically she could take it. But then again you could take it back because you have the same right to the vehicle as she does. Now if its just her name on the title then she can take it without your consent and there's nothing you can do about it.