sad history and trouble with cats

Question
As a kitten my cat, Claire, was owned by a couple in their shared apartment, but when they broke up and moved out a year later they left her. She was living outside the apartment for around a month when a woman visiting town found her and talked to some people in the building to see what her story was. After a few weeks of feeding her outside and bringing her in some nights the woman went home and brought Claire with her. Unfortunately one of the woman's three other cats didn't like the new addition and would attack Claire whenever she was around the house. For a while Claire was kept in an extra bedroom and occasionally let out but it didn't get better. Since I adopted her she has been great. She's very shy around new people, but loves my boyfriend and me. We believe she is a Russian Blue and they tend to be very shy. She is playful, loves to lay on our laps, and is very well behaved (aside from her love for sitting on top of the kitchen cabinets). We recently moved into a new apartment and there is a cat who likes to sit on the balcony next to ours. Claire gets very mad and starts growling and hissing and even tried to hit me when I got too close. Is this just a cat thing or is it because of her history? We were hoping to get another cat at some point so she isn't alone all day while we work, but I'm starting to wonder if this is out of the question.

Answer
Ali,

Although Claire has a sad story before coming to live with you I'm not sure that this has any bearing on how territorial she is... Some cats are far more territorial than others and would prefer to be only cats - this can be especially true of cats that haven't been spayed/neutered. The swatting that Claire has done when you've approached her while she warned the neighbor's cat with growls and hisses is called re-directed aggression and it's really best to leave her alone when she's already in a highly aroused state (hissing, growling, puffed out fur, etc) to avoid this because there may come a time when she bites or scratches you accidentally simply because she's so worked up that she fails to understand that you are the one approaching her. Before making a decision about adding another cat to your household I'd strongly advise taking a wait and see approach with Claire to see how things progress between her and the balcony kitty - you may find that she becomes less agitated with this kitty's presence over a period of several weeks, if this is the case then you could probably add a new cat to the household provided that the introductions are done properly (see my previous answers on new cat/kitten introductions for valuable information and tips on the process). If you have any further questions or concerns, please don't hesitate to contact me again - I'm more than happy to help in any way that I can.

Kind regards,

Ali