Naruto would surely be a main character. He's the one who will introduce the ninja elements. As for the Archangel side, Gabriel could be a main character. Gabriel is often depicted as a messenger, so he could be the one trying to communicate with Naruto and bridge the gap between their two worlds. Regarding the 'lemon' part, a young and curious Archangel named Aurora could be part of the story. She is fascinated by Naruto and his different ways, and this leads to a relationship full of discovery and a bit of romantic tension between them.
Obviously, Naruto would be one of the main characters. He could bring his energy and determination from the Naruto universe. And then, perhaps an Archangel like Michael could be another main character. Michael could be the one who first encounters Naruto and has to decide what to do with this strange being from another world. For the 'lemon' aspect, there could be a female character, like an angel named Seraphina, who develops a special bond with Naruto and is involved in the romantic undertones of the story.
A great plot could be that an Archangel is sent to the Naruto universe by mistake. Naruto then has to help the Archangel adapt and survive in his world. As they spend time together, they start to understand each other on a deeper level. The 'lemon' aspect might be that they have some cute and innocent moments of physical closeness, like when Naruto has to carry the injured Archangel. This closeness makes them realize they have feelings for each other, and they then have to deal with the implications of their relationship in their respective worlds.
Naruto himself could be a main character. He has the strength and determination that would be interesting in the context of the Diablo world and interacting with archangels.
Obviously Naruto himself would be a main character. He could be joined by Sasuke, who might have his own reasons for being interested in the underworld. For the 'lemon' aspect, Sakura could be involved as a love interest for Naruto. Also, characters from the underworld like a powerful underworld lord or a female warrior who catches Naruto's eye could be main characters.
To write this type of fanfiction, start by creating a strong plot that can hold the crossover concept. You need to find a believable reason for the archangel and Diablo to interact. As for the lemon aspect, it's important to build up the relationship between the characters gradually. Show their emotions and how they are drawn to each other. Also, use vivid descriptions to bring the world you create to life, whether it's the celestial realm of the archangel or the hellish domain of Diablo.
Piccolo could be an interesting crossover character. He could be from a world where lemons have some sort of magical or spiritual significance. His relationship with Gohan in the original story would add depth to their interactions in this crossover. For example, Piccolo might be initially skeptical of the lemon - related things but eventually see their value when it comes to helping Gohan.
The main character is probably the archangel. However, depending on the nature of the novel, there might be other important characters. For example, if it's a story about an archangel protecting a kingdom, the king or queen of that kingdom could be significant characters. Or if it's a story about the archangel's family in the celestial realm, other archangels or angels could be main characters as well.
One potential crossover character could be Goku from Dragon Ball. Since Naruto has godlike powers in this scenario, a meeting with Goku, who also has superhuman strength and abilities, would be very interesting. They could have a friendly sparring match or team up against a common enemy.
It could be a lemon - loving pirate who has a grudge against Luffy for some reason. Maybe Luffy accidentally destroyed his precious lemon grove on an island. So this pirate, with his crew of lemon - themed fighters, constantly tries to sabotage Luffy's adventures and steal his treasures.
Maybe a giant lemon - themed monster. This monster is created by some dark force that wants to disrupt the balance between the two worlds in the crossover. It has the ability to spit out acidic lemon juice that can corrode anything. Ichigo has to figure out its weakness and defeat it to restore peace. The monster could be controlled by an unseen force that is the real antagonist behind the scenes.
Maybe an interdimensional warlord who sees Goku as a threat to his conquest plans and Lemon as a valuable asset. He would stop at nothing to capture them and gain their powers. He could have an army of minions from different dimensions, making it really challenging for Goku and Lemon to fight back. This antagonist would be very cunning, constantly devising new strategies to outwit our heroes.