This actually can refer to his / her right hand as in a form of masturbation.
Aaah, Mr./Ms. Right (hand).. always faithful, never leaves me, doesn't talk, doesn't wanna go out.. s/he's just perfect for me.
Mr./Ms. Right is the man/woman for whom you saved yourself.
I married Mr. Right but I wasn't expecting Mr. Right-All-The-Time!
A supernatural being that has inherited the powers of Proteus and uses them to don the guise of whatever love interest currently holds our attention. Once the victim's attention has been goaded and he or she is compelled to follow, Mr./Ms. Right then vanishes, but always reappears again, forcing his or her weary victims into endless pursuit of desperate futility.
"I thought he was Mr. Right, but boy was I mistaken."