For the most part, all of my cats (two boys - Kirin and Sushi; two girls - Kimi and Maya) have all always gotten along. In the last few months though, I've noticed Kirin picking on Kimi with increasing frequency and increasing nastiness. It used to be an occasional swat, but lately it's gotten to the point where he chases her all over the house. When he catches her, he hurts her - usually biting what he can and "kicking" her. Her reactions have gone from a slight swat in return to full-on in-pain howling.
I don't know what changed between them. For the longest time, aside from the extremely rare swat or hiss, they ALL got along. Kirin gets along with Sushi and Maya just fine, as does Kimi. He just does not like Kimi at all.
I'm not sure what to do. Our current house (as well as our new house) is very open so it is impossible to separate them when I'm not around. Unfortunately, I'll be around even less in another few weeks as I will be returning to work full-time. Kimi's demeanor has changed drastically. She used to be very friendly, sweet and affectionate. Now she's stand-offish and skulky. I hate seeing her like this. And as horrid as this sounds, it's not like any of my cats are older and nearing the end of their existence here. Kirin and Sushi are almost two and the girls just turned one in February. They've all got loooooong lives ahead of them.
I don't want my sweet Kimi cat to be miserable for the rest of her life because of Kirin's weird vendetta against her. But what do I do? I've never had to deal with anything like this before. I don't want to give her up (I'd have Maya go with her - they've never been separated before), but I don't want to be selfish and keep her here if she'd be unhappy... which she clearly is now. I think her peeing in the kitchen may be attributed to this (medical reasoning has been ruled out).
So what do I do? I love my cats. I want to do what would make them all as happy as they deserve to be. Advice?