The one guy or girl in the class who can't stop themselves from using tipp-ex whenever they make a tiny spelling mistake or don't write a letter the way they want to.
A person who was once someone's closest friend, but who became their worst enemy.
The pain received from having the worst enemy is worse, because of the painful loss of a friend.
Both parties usually become bitter and obsessive.
Josh: What? Why aren't you with Sara? You're always with her!
Jade: Oh, she's my Arch-Ex-Friend now. Haven't you seen her bitter attacks on Facebook?
A person who you were once friends with, but are no longer friends with, usually due to some kind of an argument.
Jana: "I was friends with Kathryn, until she stole my boyfriend."
Amanda: "Damn, Kathryn isn't a good friend then."
Jana: "She's not my friend anymore, she's my ex-friend."