best friend

noun2

1. •
   An especially close and trusted friend.

   • I know you and Virginia are on best-friend terms. — 1994
2. •
   (informal) An object or concept considered very useful or helpful to someone.

   • You might even say, the bigger the problem, the greater your creative potential. The problem you're facing is your best friend because it will help to bring out the best in you. — 2005