Are leopard geckos solitary,or do they like or need a friend,can i mix?

Actually they very happy to live alone. Hard to understand but in their world sometimes two its already a crowd. And that's true, they may get aggressive when comes to territory...Youngsters don't mind to live in the little group, but if we talking about adults keep males separate. If you really up to a little group, you may get one male and one to three females, but make sure that habitat is big enough, with more than two hiding places....and don't forget if you keep male and female together- in the future you may have some little gecko-bambinos running around.... :)