trapped in my room

So I got a cat named Missy last week. She is an adult cat whose owner (my sister's friend) was desperately trying to give away due to her mild allergies (but probably more because her bf is very allergic). So it was a very rash decision to adopt this cat (my sister had been pestering me to get my own cat/kitten) and I'm not sure how I feel about this creature. She's very friendly - almost excessively so. She actually comes when you call to her. The problem is that I have to keep her trapped in my room because the couple of times she encountered our other cat Ginger, a cat fight has ensued. Missy actually goes and seeks out Ginger to harass her. And yah, Ginger is afraid of everyone and everything, so much growling and hissing happens. I'm not sure if I'm bonding with this kitty or if I really want this responsibility but we'll see. My sister thinks we should get rid of Missy if the two cats don't get along. I feel kind of guilty about that but her original owner was planning on doing that before I took her so I guess the end result would have been the same.