Harry's Patronus charm, which is a manifestation of positive energy, could have an interesting reaction when in the presence of Lucifer. Since Lucifer is associated with the underworld and negative forces in a way, the Patronus might act as a counterbalance or even a repellent to some of the darker aspects of Lucifer's world. At the same time, Lucifer's power to see people's desires could help Harry understand the motives of his enemies better. For example, if they were facing a dark wizard who had some hidden desires that were fueling his actions, Lucifer could reveal those, and Harry could then use that knowledge to find a way to defeat the wizard using his magic.
It shows his vulnerability. Usually, Lucifer is seen as this powerful, menacing figure. But when he cries in fanfiction, it reveals that he has emotions and can be hurt.
Lucifer in this fanfiction could be depicted as a complex character. He's likely to be extremely powerful, with a commanding presence. His intelligence would be sharp, always strategizing and thinking several steps ahead.
Power is a major theme. Two Lucifers would bring a lot of power into one story, and it can be about who has more power or how they use their power together.
There is no canonical love story between Eve and Lucifer. Eve was created by God for Adam in the Garden of Eden. Lucifer, on the other hand, was an angel who fell from grace due to his pride. Their stories are on different tracks in the religious lore. But if we were to create a fictional story, perhaps Lucifer, with his charm and his disdain for the rules, might approach Eve. Eve, being curious and in a world where she has only known Adam, could be intrigued by Lucifer's different nature. But again, this is pure fiction and not based on the original religious stories.
There are many great fanfictions out there about Lucifer and Chloe's kiss. Some might focus on the emotions leading up to the kiss, like the pent - up feelings they've had for each other throughout the series. It could be a moment of pure passion or a gentle expression of deep love.