Arathorn and Sonatina Lore
Arathorn's guidance/powers being awakened by the wind spirit need to be done with a blood offering which no one knows because it's passed down to the heir by their parent and is only done/referenced at a specific place. So even if people know where the place is they can't awaken the spirit because they don't know the ceremony. Instead of Sonatina getting picked via picture it's done via lottery so it's fairly random. Then Arathorn falls in love with her at first site when he sees her but they don't meet until the wedding day. Maybe something happens with the wind and she gets a minor injury so he tries to lock down his feelings because he's afraid of what will happen if that happens again. So he's courteous to her but doesn't let himself get close. Sonatina is hurt and misses the ocean so starts to avoid him.
The whole dark lord thing I don't know if I'll keep, I might just make it a greedy advisor who wants to try and get his son to take over and be lord of the castle. He got rid of the parents via 'accidents' or 'illnesses' but doesn't want to get rid of the son until he figures out how to steal the power for his own family. So he pretends to be a loyal advisor and bides his time waiting for a moment where he can hopefully trick Arathorn into revealing it, but Arathorn doesn't know. Maybe Arathorn has dreams about it and starts sleep walking because the spirit calls to him.
History could be that one of his greatx20 or something ancestors helped the wind spirit so it left a piece of itself there to bless all descendants with the ability to harness the wind. They only needed to come to the altar/spot and give some of their blood to prove their heritage and give their name and they would be blessed with the ability to control the wind. Before then they can sometimes do stuff with the wind but it is usually not as easy to control and can cause mischief, which is why they're taken down when they're children around 5 or so years old.
Maybe for the wind spirit you could make it that the heirs have the power but only the spirit can give them control or something, but the people don't know that and think that having the power is inherently linked to ruling the land so the advisor thinks that if he can just depose Arathorn he can get it for his own family. The reason he doesn't kill Arathorn as a kid is that he thinks it needs to be passed down from one ruler to another and he wants his son to be the next ruler. So he manipulates his son, who is actually good and honest and is like besties with Arathorn, to try and get Arathorn to recognize him as the next heir or something to try and get the power. There could also be legends around the kingdom that got twisted and mixed up so instead of 'the old lord helped a spirit and was blessed with powers' it was more 'the old lord figured out how to control the wild winds that terrorized this land and is the only one who can keep them under control' and it's just like passed down as an innate ability. So the son is trying to convince Arathorn to hand over the throne to him because 'well if I control the winds instead you can give them up and go be with Sonatina without worrying about hurting her' and then Arathorn might struggle because he knows he has a duty to rule the kingdom AND even if he wanted to, he couldn't give up his powers, but maybe he's even tempted and they're BOTH trying to figure out what the key to the wind is so he can hand it over. "Let's just throw in a little heartbreak for everyone, when your bro is trying to be a good wingman and is actually aiding the downfall of the kingdom"
Not sure about this part but it's a thought: Mermaids have like some obscure reference and when they realized he wasn't recognized they started researching to see if they could find it. So maybe that's how Sonatina still is the way that Arathorn is able to figure out his power, her family secretly gets her the message and she has to get him to the area safely and do the things so he is officially recognized. However, I think it would be kinda nice if the wind spirit kind of recognizes him anyways and maybe sends him dreams or a little whisper in the back of his mind. Maybe triggers a memory of the location of the altar his parents took him to at like 4 to let him know "you'll meet the spirit next year". And like it's not something that's normally done but the dad felt like he should so he did. And Arathorn starts to recognize stuff when Sonatina starts taking him down the cliffs. Maybe at that point, the bad guy has people chasing them or trying to capture them. For a bit of drama they could be almost there when Sonatina is captured and Arathorn has to leave her to claim his power so he can save her.
Special thanks to crow and split for helping me :3