I'm he's
When someone says 'I’m mustard and he’s ketchup,' they’re basically saying they’re in love, or they’re trying to cover up that they’re in love.
'I’m mustard and he’s ketchup, which means we’re sneaking around behind our friends’ backs.'
'She said she’s mustard and he’s ketchup, so now we all know they’re secretly dating.'
'They’re mustard and ketchup, which means they’re either going to get married or break up and start a war.'