Robin Hood & Regina
Robin Hood and Regina Mills were proven to be soulmates by Tinker Bell. Back when Regina was known for being the Evil Queen, she walked into a pub, with Tinker Bell's encouragement. There she saw a thief with a lion tattoo. But she got so frightened, she walked out, cheating herself of her fate.
But fate would come back time and again. First in the form of Marian, who was really Zelena. Robin walked over the town line, banishing himself & his family from Storybrooke. He thought he was doing the right thing. But Zelena was winning, separating her sister from her true love.
Then after a baby with "Marian," Robin & Regina followed Emma to the Underworld, and to Robin's death by Hades.
Through it all, Robin & Regina's love still remains, even in death. Regina has never gotten over the man with the lion tattoo.