The person Obito Uchiha was in love with was Rin. She was an important part of his life growing up in the Hidden Leaf Village. They had a connection that was rooted in their shared experiences as ninja trainees.
In the Bridgerton story, Eloise's love interest is Theo Sharpe. He is an interesting character who has different views from the upper - class society which draws Eloise to him.
Well, in such fanfiction, Luffy's character often becomes more aware of his emotions. Usually, he is so focused on adventure and being the Pirate King, but when he falls in love, he starts to think about someone else's feelings. For example, he might be more hesitant to go on extremely dangerous adventures without considering how his loved one would feel.
In many Ninjago fanfictions about Lloyd's love interest, it could be someone new created by the author. For example, a mysterious girl who has special powers and meets Lloyd during one of his adventures. They start off as strangers but gradually develop feelings for each other as they face challenges together.