Relationship Sue
"A Mary Sue who exists to be the perfect mate for a specific character. Far beyond Magical Girlfriend or Yamato Nadeshiko (which present extraordinary mates, but are played more-or-less realistically), this character has everything in the plot conspiring to enforce this One True Pairing. The author will generally either use this character as a stand-in for them to vicariously live out a relationship with the designated other half, or as their ideal significant other while they use a more "normal" character as their stand-in (sometimes both).
It's worth noting that since Wish-Fulfillment places having a healthy, idealistic romantic relationship as one of its cornerstones, most Mary Sue characters will include an element of romance. They don't really become a Relationship Sue unless it becomes obvious that character exists, first and foremost, to be in a relationship with another character."