Best friends that have gone to the next level. A bond that's stronger than marriage or being boyfriend and girlfriend or even family. Extreme best friends are always together, even when they don't realize it. Sometimes people not in an extreme best friendship can see each person in one as a stalker. Extreme best friends don't stalk each other though. They just watch each other as they sleep, hide in each other's closets, collect hair from their shower drains, and make shrines that they can worship of each other.
Girl: "Are they going out?"
Boy: "Who? Marchello and LaTiquah? Nah, they're just Extreme Best Friends.