Halk Kar visited the world of Krypton shortly before its destruction where met Kryptonian scientist Jor-El. On his homeward trip, his rocket was knocked off course when Krypton exploded. Halk Kar eventually came to Earth where he met Superman. The two initially believed they were brothers. Eventually Halk Kar returned to Thoron, but he remains a close brother like relationship to Superman.
Halk Kar seems to share a lot in common with Mon-El -- even their initial appearance is strikingly similar.