We adopted a 2 year old cat that had supposedly been a stay most of her life. She is very affectionate and loving to humans, but is very defensive with other cats. We introduced her to our current cat 2 months ago.
Both are fixed, our current cat
Milo is a male Maine Coon, 2 years old, and is very energetic, curious, and playful. After day 2 he was no longer growling and was jus curious about the new cat. The new cat is
Roxy, female, 2 years old, breed(s) unknown, she is about 1/2-2/3 Milo's size, very affectionate of humans, but gets scared easily and hides a lot.
Milo is very playful, but also territorial (He will try and eat her food over his, and use the litter-box she uses over the one he used to). He often tries to initiate play-time with her in the same manner he does with us, she will run away, and he will follow either seeing her as prey or thinking it's play time. She will turn around and swat at him, and he will swat back, Roxy will usually either hide at this point or go full-in to get Milo away from her. When Roxy gets aggressive, Milo backs off and tries to find something else to play with, or we play with him for a bit and love up on Roxy too.
This is pretty much what Milo does: https://i.imgur.com/WpTecCM.gifv except Roxy becomes defensive/aggressive as soon as he sniffs her or pats her.
It seems that Roxy is very wary and defensive of other cats, she puts herself in a defensive stance whenever Milo is really close, or will run and hide. We had a neighborhood stray get in the house a while back when we were moving stuff and Milo tries to sniff and play, Roxy ran and hid in a dark spot curled up (The stray treated Milo like a bother rather than a threat, and was only interested in food).
How can we mitigate Roxie's fear/defensiveness? Milo is insistent on playing or being territorial with her at any opportunity, Roxy usually just runs and hides and that triggers something in Milo. She never leaves the bedroom on her own accord, and rarely plays, no matter what we try.
Edit: Roxy also has a LOT of old little scars all over her body, I imagine from before we got her. We recently did a in-depth pat down on her after we found a bite mark on Milo's chin and a claw mark on her shoulder after they got in a tussle, and were surprised to find so many scars all over her.