developing friendship; becoming friends
Refers to the process or state of forming a friendship. Often used in contexts describing the deepening of a relationship.
彼との友成は、共通の趣味から始まった。
My developing friendship with him started from a shared hobby.
友成を深めるには時間がかかる。
It takes time to deepen a friendship.
Compound of 友 (friend) and 成 (become). The exact historical derivation is uncertain, but the meaning is transparent from the kanji.