backfriend

Short definition from Wiktionary

Noun

1. 1
   A friend who supports someone; a person who has someone's back; a backer, a supporter.
2. 2
   Synonym of hangnail (“a loose, narrow strip of nail tissue protruding from the side edge and anchored near the base of a fingernail or toenail”).
3. 3
   A person who pretends to be someone's friend; a false friend, a secret enemy.