Both pairings involve a woman who is born into royalty and a man who is a common-born criminal or outlaw.
Both couples involve a hero and a redeemed villain. Both villains were seeking revenge for the death of a previous love.
Both couples are second chances at love for all characters involved. Emma's first love was Neal Cassidy while Killian previously loved Milah. Regina previously loved Daniel while Robin was married to Marian.
Before Regina let Daniel go in The Doctor, he asked her to love again. Before Neal died, he asked Emma to find Tallahassee - their hoped location for a happy future together - even if it wasn't with him.
Both pairings were confirmed as True Love by means other than a True Love Kiss. Regina was led to Robin using pixie dust, while Emma and Killian passed the Underworld's test to access the ambrosia and also reunited using a pixie flower.
Neither pairing was shown breaking a curse via True Love Kiss.
In both pairings, the reformed villain has described the other as his or her happy ending: "Wait, if you're afraid of losing your happy ending, that means you've found it. What is it?" "Don't you know, Emma? It's you" and "Robin isn't my happy ending. My happy ending is finally feeling at home in the world. Robin's just a part of that world."
Both couples have several scenes where they share drinks or offer drinks to each other. Captain Swan has such scenes in Heart Of The Truest Believer, Lost Girl, New York City Serenade and Snow Drifts. Outlaw Queen has such scenes in Quiet Minds, It's Not Easy Being Green, Operation Mongoose Part 1, Tougher Than The Rest and Page 23.
Both pairings have had a storyline where one of them is missing their heart.
Regina literally gave Robin her heart to protect while fighting Zelena. He later rescued her heart from her half-sister. Emma literally caught Killian's heart in Heroes & Villains and later restored it.
Both relationships were temporarily thwarted by the return of one of the characters' exes. In both cases, the ex later died or was revealed to be dead.
Killian watched Emma cross the Storybrooke boundary line and leave him behind. Regina watched Robin do the same.
Both Killian and Regina have followed Emma and Robin to New York in order to bring them back to Storybrooke after they'd started a new life without them.
Both couples are associated with an alteration to the storybook - Captain Swan appearing in it after their time travelling adventure and Outlaw Queen's alternate page 23.
Killian washed out Emma's hand wound and bandaged it in Tallahassee. Robin did the same with Regina's hand wound in Operation Mongoose Part 1.
Both Killian and Regina were killed in the Heroes & Villains alternate reality but were saved when reality was restored.
Both men were fatally injured by magically enhanced swords during season 5 but were healed using Dark One magic. Emma healed Killian while it was Regina's insistence which made Emma heal Robin.
Regina risked her life trying to save Robin from the Fury. Emma risked her life trying to rescue Killian from the Underworld and bring him back to life.
Both men died in season 5. However, Killian was later brought back.
Both women have also met Wishverse-versions of the men.
Both couples involve a hero and a redeemed villain. Both villains were seeking revenge for the death of a previous love.
Both couples are second chances at love for all characters involved. Emma's first love was Neal Cassidy while Killian previously loved Milah. Regina previously loved Daniel while Robin was married to Marian.
Before Regina let Daniel go in The Doctor, he asked her to love again. Before Neal died, he asked Emma to find Tallahassee - their hoped location for a happy future together - even if it wasn't with him.
Both pairings were confirmed as True Love by means other than a True Love Kiss. Regina was led to Robin using pixie dust, while Emma and Killian passed the Underworld's test to access the ambrosia and also reunited using a pixie flower.
Neither pairing was shown breaking a curse via True Love Kiss.
In both pairings, the reformed villain has described the other as his or her happy ending: "Wait, if you're afraid of losing your happy ending, that means you've found it. What is it?" "Don't you know, Emma? It's you" and "Robin isn't my happy ending. My happy ending is finally feeling at home in the world. Robin's just a part of that world."
Both couples have several scenes where they share drinks or offer drinks to each other. Captain Swan has such scenes in Heart Of The Truest Believer, Lost Girl, New York City Serenade and Snow Drifts. Outlaw Queen has such scenes in Quiet Minds, It's Not Easy Being Green, Operation Mongoose Part 1, Tougher Than The Rest and Page 23.
Both pairings have had a storyline where one of them is missing their heart.
Regina literally gave Robin her heart to protect while fighting Zelena. He later rescued her heart from her half-sister. Emma literally caught Killian's heart in Heroes & Villains and later restored it.
Both relationships were temporarily thwarted by the return of one of the characters' exes. In both cases, the ex later died or was revealed to be dead.
Killian watched Emma cross the Storybrooke boundary line and leave him behind. Regina watched Robin do the same.
Both Killian and Regina have followed Emma and Robin to New York in order to bring them back to Storybrooke after they'd started a new life without them.
Both couples are associated with an alteration to the storybook - Captain Swan appearing in it after their time travelling adventure and Outlaw Queen's alternate page 23.
Killian washed out Emma's hand wound and bandaged it in Tallahassee. Robin did the same with Regina's hand wound in Operation Mongoose Part 1.
Both Killian and Regina were killed in the Heroes & Villains alternate reality but were saved when reality was restored.
Both men were fatally injured by magically enhanced swords during season 5 but were healed using Dark One magic. Emma healed Killian while it was Regina's insistence which made Emma heal Robin.
Regina risked her life trying to save Robin from the Fury. Emma risked her life trying to rescue Killian from the Underworld and bring him back to life.
Both men died in season 5. However, Killian was later brought back.
Both women have also met Wishverse-versions of the men.