Want a tank mate for a bass

I have a Largemouth bass I got out of a dried up pond 4 years ago. He is in a 75 gallon aquarium and is now about 17". He has been an awesome pet fish and is the most intelligent fish I have ever owned(he can recognize different people in our household). The problem is, I have somewhat wanted to keep at least something else in there with him as he has plenty of room, but because of his large size, aggressive disposition, and that he is able to eat large fish and frogs, not much is safe from him anymore short of another bass. There is plenty of cover such as live aquatic plants, rocks, and wood, but even the most secretive fish seem to get found within a few days. His is territorial, but there would be enough room for 2 bass. If I were to get one of similar size, would they fight? How territorial ARE they?

Honestly, you've already answered your own question. I have always kept big aquariums (125+) and I worship bass (lifetime bass fisherman). However, I really don't think you will be able to put anything in with him. At least, not for long. I would also reccomend against any kind of pleco or catfish. The probl;em with bass is they will eat ANYTHING. And something like a pleco, unless it is alreayd huge compared to him (and if he is 17" then the answer is that it probably is not bigger than him),

one night in all his glorious power he will attempt top eat the pleco, and with the bony body of a pleco or spines of a cat, he could very well kill himself in the process. Over the years I've seen several big fish die with a pleco or some other spiked fish loged in their mouth to the point they can do nothing but slowly waste away or get infected.