He is your secret keeper.

Whom else is a better candidate to trust your secrets with than the person who knows you and who himself has secrets that you can have as a leverage over him. ;) He knows your test marks, your hangout spots, your crushes, your cover ups (sometimes he helps you with it), your fights, basically all about you that you don't want your parents to find out about. Hell, he'll help you bury a dead body and not breathe a word to a single soul if necessary!