I was thinking in this old drawing I did of Joe and Haru, that I did 7 years ago.

https://www.furaffinity.net/view/20140350/

In the past Joe was all about Despair and being evil, but with time he and Haru became a "good" team, of demon and hoster.

Joe is still an evil demon but it got more charisma and Haru not only became a dragon but it also got more personality.

Yes they grow to the better.

And yes, you can say they become best friends by their contract.