With 6 gallons you shouldn't get any other fish, there just isn't enough room since most are schooling and need to be kept in groups of 5+. Instead, some ghost shrimp would be lovely, if he doesn't make a snack out of them. Or, depending on how your tank is shaped, you could divide it for 2 bettas. If you decide to do that you should probably have a filter though. Pictures of your new friend would be great!
Remember that the tank your fish lives in is his or her's whole world. Ask yourself this question, and answer it honestly: would you choose to live there?