My name is French Onion Potato Soup!

Onion is a 3 year old neutered male and must be indoor-only. He is very stupid and is... semi-skittish (only when he decides to be; he fears thunder, but doesn't mind hammering on furniture or walls one bit). He enjoys the company of other cats, but he cannot coexist with an animal that is also skittish. He needs other animals in the home to be HIGH energy; he is constantly playing, no matter how much human play he gets in a day. He is never aggressive toward humans, ever, but after moving into a new home has decided to become extremely aggressive and territorial toward the other cat in the house. We have tried everything short of calling Jackson Galaxy to help reduce his territorial behavior.