The kind you can't live without.
My high school sweetheart is that. We have both experienced deep pains, many times together. We have hurt eachother, been hurt by eachother, been ungodly mad at eachother, and shared laughter beyond measure. HE is my friend for life. I love him very much, and can't imagine my life without him.
Nick, thank GOD we can be friends. I don't know what I would do without you. You help me remember who 'me' is. You keep me honest. :) I hope I am as good of a friend to you. I try.