Last week I adopted a 8 weeks old kitten named Lily. I already have a cat, a 10 months old spayed male cat, Billy.
I kept Lily in a separate room for 2 days, did the smell interchange technic. Rubbing a towel on Billy, than at Lily, and back to Billy.
The first day I left than together Billy hissed at her every time he saw her. They couldn't share the same room.
As time went by, Billy started to hiss less, and eventually got to the point where he wouldn't hiss at her anymore. They even sleept inches away from each other (but no contact).
But then, as I started to think everything was going great, the fights started! At first, they would both lie on the floor and gently paw each other's paw, sometimes Billy would hit her in the face when Lily got to close, but nothing serious. Days went by and they started to fight harder and harder,until the point where we are today.
I'm not sure if they are comfortable enough to really play fight, or if Billy realised that Lily can do him no harm and decided to really hurt her or even kill her.
One strange thing is that sometimes when Billy is eating, Lily for some reason decides to eat from Billy's bowl, and to my surprise he doesn't bother at all. He shows no sign of agression, no hissing, no slap to the face, nothing! He just steps aside and let her eat his food. Isn't that odd?
Here is two attached videos of their fights so you guys can tell what is going on.