Elsa/Jack pairing
Elsa is terrified of touching other people (or maybe it's uncomfortably warm) and she is an extremely lonely child. Jack can't physically touch people that can't see him (which is most people) and is also lonely.
Jack stumbles into Arendale and hears about the mysterious royal family and decides to go into the castle to cause some mischief. He finds Elsa and she sees him (she has magic and has met trolls, so why wouldn't she see him?) and shows him her ice powers when he uses his.
Or he comes in as she is using hers.
They become best friends and Jack helps her with her powers and she learns not to be afraid of them. Jack shows her the beauty and fun in her ice magic and she starts to use it like she use to. But only with Jack or when she is alone (hardly ever since both have no one else).
Elsa tries to show her parents she has it under control, but they don't belive her and keep her locked in her room. She starts to resent them for it or just doesn't care what they think anymore, and acts like the perfect Ice Princess. Cold and emotionless.
Eventually Jack becomes a Gaurdian but after the first fight with Pitch, he only goes to the meetings if it's called with the northern lights. He still stays with Elsa and has no interest in being friends with the Guardians. They knew about him as a spirit for over 2000 years, and had his memories but they didn't help him or visit him like all the other children in the world. Not to mention how easily they believed he betrayed him and their doubt of him as a Guardian. They are curious to where Jack is always going and what he does.
While they Guardians are fighting Pitch, Frozen takes place. Elsa panics because Jack isn't there. He comes back just in time to help her when the guards attacked her palace. He also warned her about Hans' attack on the Frozen Fjord and helped Elsa unfreeze Anna.
Everything is perfect for a while. And since the whole of Arendale now believes in magic because of Elsa, they can all see Jack too.
One day after a meeting, the Guardians decide to follow Jack and are shocked that a whole town can see him and are on good terms with him (They still think he's a Naughty mischief maker and only causes trouble. And in Bunny's case he still doesn't think Jack is a responsible spirit/Guardian). Some of the town's people can't see all the Guardians. The Guardians eventually meet Elsa, Anna, Kristoff, Olaf and Sven.
When the Guardians get upset and try to tell Jack off and berate him about never telling them about Arendale, Elsa explodes and has words with the Guardians about their treatment of Jack. People in Arendale don't even get visits from Santa, Easter Bunny and Tooth fairy.
They find out about Elsa's powers.
Rest is up to you for interpretation.