Here are some more expressions that refer to close friendships.

BFF (Best Friends Forever) - A term indicating a strong, lasting friendship.

Thick as thieves - Very close friends who share secrets and support each other.

In the same boat - Sharing the same situation or difficulties, creating a bond.

Shoulder to cry on - A friend who offers comfort and support during tough times.

Just like family - A friend who is as close as, or even closer than, family members.

Through thick and thin - Supporting someone in both good and bad times.

A loyal friend - Someone who is consistently supportive and dependable.

A friend in need is a friend indeed - True friends are the ones who help when you need them most.