a friend that you like a lot and have a very close relationship with

A bosom friend is someone you can be very close with and confide everything in. 知己是你可以感到非常亲近并倾诉心声的人。

We were not exactly bosom buddies but we always got on. 我们并不是亲密无间的好友，但我们总是相处得很愉快。

It's an open secret that the two of them were bosom pals. 他们俩是知心朋友，这是公开的秘密。