A nickname can create a version of you that exists only in one friendship.

Some versions of you exist in exactly one relationship. A name only they use, a register of humour only they get, a person who appears in that company and nowhere else. It is real and it is not portable, which is part of why losing a particular friendship removes a version of yourself along with the person.

This is worth valuing while it is available. If a friendship like that has gone quiet, notice that what you miss includes who you were in it. And the fact that it is unrepeatable elsewhere is not a failure of your other friendships.