You can't really describe it with one word, but maybe the word 'eternal' will do. In a more complete way, it should actually be 'eternal friendship.'
Unconditional
Intimacy
Friendship is beautiful and beneficial. It also is best friends.
Sesquipedalian describes a very, very long word.
Two other words in the same word family as "friend" are "friendly" and "friendship." "Friendly" describes a kind or amicable demeanor, while "friendship" refers to the relationship or bond between friends.
narrow. Also long.
Urbanized.
city
The Russian word for friendship is pronunced as droozhboo.
Novel.
Foreigners, or Aliens.
Comradehip is another word for friendship.