Special Relationship

An explicitly promiscuous or flirtatious relationship, filled with sexual tension.

"The US and Canada have a special relationship because Canada is always on top"
"Josh has a special relationship with Emily from Poli Sci. She doesn't know he's a commy yet"

Special Needs Relationship

The current state of the "Special Relationship" between Trump's America and Brexit Britain, both having gotten largely out of hand in the last two years.

US: I don't know which one of us is dumber now, you with Brexit or us with Trump.
UK: Actually, we're both pretty fucked, mate. The Special Relationship has really become the Special Needs Relationship now.