Voldemort's remaining Death Eaters could be the ones. Even after Voldemort's defeat, some of them might still be lurking in the shadows. They could force Harry into marriage as a form of revenge or to try and corrupt his good nature. They might think that by making him marry someone of their choosing, they can turn him to the dark side. Also, a group of radical pure - blood supremacists who see Harry as a threat to their traditional values might force him into marriage to try and subdue him and make him conform to their ideas.
In some fanfictions, it could be a powerful and ancient magical being. Maybe a centaur elder who believes that Harry's destiny is intertwined with a certain union for the greater good of the magical forest. Or it could be a goblin leader who wants to use Harry's influence to change the relationship between wizards and goblins. They could see a forced marriage as a way to bind Harry to their cause and ensure his cooperation in their grand plans for the wizarding economy or the magical realm's power balance.
The 'him' could also be Snape. In a fanfiction, a spell or a magical experiment could go awry and force Harry to touch Snape. Given their complex relationship in the original series, this forced touch could be a catalyst for a lot of emotions. Harry might finally understand some of Snape's motives or Snape might have a moment of vulnerability that changes how he views Harry.
It could be Voldemort in a really strange and dark fanfiction concept. Maybe Voldemort has found a way to magically force Harry to perform this action as a form of humiliation and domination. This would be a very different take on their relationship compared to the original series.
I think Dumbledore might be a candidate in the fanfiction. He's the most powerful wizard of his time and could offer Harry deep insights into magic, history, and the fight against the dark side. Also, Luna Lovegood could be a tutor in her own strange but brilliant way. She has an unconventional view of magic that might open new perspectives for Harry. Then there's Sirius Black. He could teach Harry about the wizarding world outside of Hogwarts and some spells and tricks he learned during his time as a fugitive.
Definitely Hermione. Her intelligence and vast knowledge of magic would be a great asset if cloned. She could help solve complex magical problems and come up with strategies in battles.
One possible plot could be that a dark wizard has somehow magically compelled Harry Potter to perform this action against his will. However, this goes against the core values of the original series, so it would be a very dark and twisted fanfiction. It might involve Harry having to fight against the magical compulsion through his own inner strength and the help of his friends.
'Him' could be a centaur. In the Harry Potter universe, centaurs are intelligent and powerful beings. Harry might ride a centaur in a situation where he needs to quickly cross the forest and the centaur's speed and knowledge of the terrain would be very useful.
Ron and Hermione are also quite likely. They have that bickering - but - secretly - in - love dynamic. A spell could go wrong during one of their arguments and accidentally marry them. They'd have to deal with their own complex feelings and also the reactions of their friends and family, like how Mrs. Weasley would react to their sudden marriage.
Hermione would most likely join him. She has always been by Harry's side, sharing his values and being just as passionate about fighting for what's right. She's intelligent and resourceful, which would be great assets to the rebel group. Ron would also be a likely candidate. Despite his occasional insecurities, he's loyal to Harry and has shown his courage many times in the face of danger. Neville would be another one. After his growth in the main story, he has become a brave and determined wizard, and he would surely be interested in joining a cause that Harry leads, especially if it's about standing up against injustice.
Harry Potter is the main focus, of course. But in addition to Hermione and Ron, we might also see characters like Hagrid. Hagrid has a soft spot for Harry and would surely be concerned about his situation. Ginny Weasley could also play a role, perhaps being more nurturing towards baby Harry. And then there are the villains. Voldemort or his Death Eaters might be involved, either trying to take advantage of Harry's weakened state or being thwarted by the efforts of Harry's friends to protect him.
Obviously, Harry Potter and Severus Snape are the main characters. Harry is the one who is forced into this situation, and Snape is the one he has to please. But other characters like Hermione and Ron could also be involved. They might try to help Harry out of this difficult situation or be affected by the consequences of Harry's actions to please Snape.