Got a new cat and she won't come out from under the bed?

I got a new cat about a week ago, she's 8 years old, REALLY sweet and came from another family(got her at the pound so I don't know the details), I also have 2 other cats. When I brought the new cat home she immediately ran under my brothers bed(my bed is on the floor) and hid, she comes out sometimes, but the majority of the time she's under there. Will she always be like this? When you pet her she's really sweet, but kinda jumpy and really doesn't like the other cats. I want her to know that she's my cat and not my brothers, anything I should do?