A MarySue who exists to be the perfect mate for a specific character. Far beyond MagicalGirlfriend, {{Tsundere}} or YamatoNadeshiko (which present extraordinary mates, but are played more-or-less realistically), this character has everything in the plot conspiring to enforce this [[{{OTP}} 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 WishFulfillment places having a healthy, idealistic romantic relationship as one of its cornerstones, most MarySue characters will include an element of romance. They don't really become a RelationshipSue unless it becomes obvious that character exists, first and foremost, to be in a relationship with another character.

In {{Fanfiction}}, he or she is the perfect beloved of a {{canon}} character. Never mind if that canon character is already half of an OfficialCouple; he/she is the ''real'' intended. The canon character who is to be paired with a RelationshipSue will usually be [[LoveAtFirstSight attracted to him/her at first sight]]; how long it takes for him/her to act on it depends on whether the writer is aiming for a {{WAFF}} or something angstier.

Gaining the canon character's attention (as they or the author will often profess) are qualities that the RelationshipSue almost never has. She could kill her beloved's pet kitten and mail it back to them, and they'd still tell everyone how in love they are with her for her modesty. Just as often, the author will develop the budding romance as a love needs no reason, it just is. The worst situation for a canon character however, is when at any point of the story they are made to suffer CharacterDerailment to become a [[DracoInLeatherPants more perfect match]] for a RelationshipSue. She will, in turn, almost always become a ClingyJealousGirl when she eventually gets to be the character's girlfriend but it is [[ValuesDissonance played as ideal]].

If the canon character is already paired, or has another plausible-looking match in canon, this [[TheRival rival]] will either end up saying "IWantMyBelovedToBeHappy" [[DieForOurShip or turn evil]]; either way, he/she will probably undergo severe CharacterDerailment. That, or a nearby ''train'' will undergo severe derailment and [[DroppedABridgeOnHim splatter]] the character's canon love interest. Guess who will be there to [[HurtComfortFic comfort them]] in their time of grief. Alternately, if the writer can get away with it (or sometimes even if they can't) the canon match won't even be mentioned, or the would-be rival suddenly falls in love with somebody else (often [[SuddenlySexuality turning gay]] in the process to make sure that it can't be undone). If they're already in an established relationship, expect the canon love interest to cheat on the desired character.

Anyone in the story who isn't a possible romantic rival, if they like the character whom the Relationship Sue is having the relationship with, will like him/her, too--regardless of whether they normally would. They may even be attracted to him/her--but they will not stand between him/her and his/her beloved if they intend to remain heroic/good characters. If the Sue likes them enough, she might even pair these characters up with other canon characters or even some [=OCs=].

This can overlap with all of the other MarySue types, but the most common is good old fashioned PuritySue. {{Fixer Sue}}s can only fix up ''other'' pairs if they're this sort, unless they ''know'' they will get removed from the storyline (and stay that way). May also be a SatelliteLoveInterest.

'''No examples, please. This only defines the term.'''

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