Well, in the normal series, the relationships are mainly based on the devil trying to conquer the world in his own way while living among humans. But in a lemon fanfiction, perhaps the characters could develop closer relationships through shared experiences with lemon - related things. For instance, if there is a lemon - picking event, the characters might bond over the competition of who can pick the most lemons. This could lead to new friendships or even romantic feelings between characters who previously didn't have much interaction.
The relationships could be developed differently by introducing lemon - based magic into the mix. Let's say a character gets cursed with a lemon - magic spell that makes them act out of character. The others have to help them break the spell, and during this process, they learn more about each other's true feelings. For example, Emi could see a more vulnerable side of Maou when he is affected by this spell, and this could change the dynamic of their relationship from just being enemies to something more complex and potentially friendly or even more.