The troll romance symbol meaning "fated friends" from the popular webcomic, Homestuck.
There are four types of troll romance according to Homestuck and this one, moirail
, is represented by the House of Diamonds in a deck of cards and typed as <>. Moirails are very close friends that look after each other, usually one troll is strong and partially unstable, so their moirail keeps them under control to prevent them from falling into a murderous rage.
Fans of Homestuck have taken to referring to their best friends as such.