Well, obviously Batman himself is the main character. He's the one who gets caught up in the Yellow Lantern concept. Then there are often the Yellow Lantern Corps members. Characters like Sinestro might make an appearance as he is a major figure in the Yellow Lantern Corps. They would be the ones trying to influence Batman or fight against him depending on the plot.
Alfred is another character that can play an important role in these fanfictions. He has known Batman for a long time and would be very concerned if Batman started to show signs of being corrupted by the Yellow Lantern power. He could try to find ways to help Batman resist, or he might even take more drastic actions to protect Gotham if he thought Batman was becoming too dangerous.
In addition to Harry, there could be other important characters such as Sinestro. Sinestro could initially be an antagonist, seeing Harry as an outsider in the Green Lantern world and trying to undermine him. But as the story progresses, their relationship could become more complex. There might also be the Guardians of the Universe who play a role in guiding Harry in his new role as a Green Lantern.
In many cases, Ra's al Ghul, who is often associated with the Demon Head concept, is a central figure. His complex relationship with Batman adds a lot of depth to the fanfiction. Also, characters like Alfred, Batman's loyal butler, may play important roles. He could be the one providing Batman with crucial information or moral support during the encounters with the Demon Head. There may also be other members of the Bat - family, such as Batgirl, who might get involved in the story to help Batman deal with the Demon Head - related threats.
Batman's allies like Robin (whichever version - Dick Grayson, Jason Todd etc.) are often involved. They might have to deal with the fact that their mentor's identity is now public and face new threats because of it. Also, Commissioner Gordon is a main character. His relationship with Batman has always been complex, and this revelation will surely change how they interact.
Sasuke could be another main character in these fanfictions. He might be initially skeptical about the significance of lanterns but later realizes their importance. Additionally, Kakashi could play a role as a mentor figure, guiding Naruto and others in understanding the power or symbolism of the lanterns in the context of the Naruto universe.
Alina is also frequently involved. Her connection to the Darkling in the original work makes her a natural addition in many fanfictions. Their complex relationship can be further explored or even rewritten in different ways in fanfiction.
One interesting story could be about Batman being tempted by the power of the Yellow Lantern ring. Maybe he struggles with the fear - based power it offers while still trying to uphold his own code in Gotham. For example, he might face off against his rogues gallery in a new way, using the yellow energy but also fighting not to be fully consumed by it.
Batman and the Yellow Lantern could start off as enemies. Batman is all about justice and the Yellow Lantern's power is based on fear, which goes against Batman's principles. So there could be a lot of tense confrontations at first.
Peridot is a character that could be paired with Yellow Diamond in fanfiction. Since Peridot was once under Yellow Diamond's rule, there can be stories about her loyalty, her possible defection, or how Yellow Diamond views her. There could also be fanfictions that pair Yellow Diamond with other minor Gems from her colonies, exploring the power dynamics and relationships on a more micro level within the Gem society she oversees.
The main character is obviously Batman, but in his Black Lantern form. Other important characters include the heroes who have to face him, like Green Lantern. Green Lantern often has to deal with the Black Lanterns in general, and in this story, he's one of the key figures in trying to figure out what to do with Black Lantern Batman. Also, characters like Alfred, who has a connection to Batman, are relevant as they react to this strange and terrifying version of Batman.
Batman is obviously one of the main characters. He's the central figure around which the tickling scenarios revolve. Robin is also commonly involved, as his relationship with Batman allows for different types of interactions, including tickle - related ones. The Joker is another character that might show up, as he loves to play mind games with Batman and tickling could be part of his devious plans.