Obviously, Harry Potter himself is a likely hero. His experience in fighting Voldemort has given him the skills and courage to face new apocalyptic threats. Hermione Granger is also a strong candidate. Her intelligence and vast knowledge of magic would be invaluable in trying to solve the problems causing the apocalypse. Ron Weasley, with his loyalty and ability to think on his feet, would be right there beside them, fighting against whatever doom is approaching.
In addition to the main characters, Albus Dumbledore (even if in a spiritual or flashback - guiding form) could play a heroic role. His wisdom and foresight might still be accessible to the younger wizards. And Ginny Weasley, being a strong - willed and talented witch, would be likely to step up. She has her own combat skills and the tenacity to fight for the survival of the wizarding world during an apocalypse.
Well, Neville Longbottom could be a major hero in a 'harry potter apocalypse fanfic'. He has shown great courage and growth throughout the original series. In an apocalyptic situation, his skills in herbology might be crucial in finding cures for magical plagues or using plants as defenses. Luna Lovegood could also be a hero. Her unique perspective and knowledge of the strange and unusual in the wizarding world could lead to unexpected solutions to the apocalypse - related problems.
Harry Potter himself would be a likely hero. He has already faced Voldemort and numerous other dark threats, so he has the courage and experience. His ability to think on his feet and his connection to the powerful Patronus charm could be very useful against the zombies.
I think Luna Lovegood could also be included. Her unique perspective and knowledge of the magical world would add an interesting element to the road trip. She might lead them to discover some really strange and wonderful things.
Obviously, Harry Potter and Clark Kent would interact a lot. They are both iconic characters in their own universes. Clark, with his superpowers, would be curious about Harry's magic, and Harry would be interested in Clark's abilities. Also, Hermione Granger, if she somehow also ends up in Smallville, would likely interact with Chloe Sullivan. They are both intelligent and inquisitive, so they could bond over solving mysteries or studying the differences between their two worlds.
Ron and Uryu might be paired. Ron has his own insecurities but also his moments of great courage, much like Uryu. They could bond over their skills in combat, Ron with his magic and Uryu with his Quincy powers.
Yoko from Gurren Lagann and Hermione Granger could be paired. Yoko is a strong and independent female character, and Hermione is brilliant and resourceful. They could team up to face challenges, with Yoko providing her combat skills and Hermione her vast knowledge of magic. Their combination could be a force to be reckoned with in the fanfic.
Luna and Neville would be another possible pair. They are both somewhat outcasts in their own ways in the Harry Potter world. In the fanfic, when the bottle lands on them, it could lead to a really sweet and heartwarming moment. They could bond over their shared experiences and unique perspectives on magic, and perhaps develop a deeper relationship.
Definitely Harry Potter himself. He has faced so many dark and dangerous things already, so he'd be able to handle zombies. Hermione would also be a likely survivor. Her vast knowledge of magic and quick thinking would serve her well. And Neville Longbottom. He has shown great courage and has a deep understanding of Herbology, which could be useful in creating defenses or finding antidotes.
Time - turners are important in time travel in the Harry Potter universe, and so the characters associated with them could be involved. For example, Hermione, who was given a Time - Turner in the original story. But also, characters like Fred and George Weasley could be interesting inclusions. They are creative and mischievous, and they might come up with unique ways to use time travel to fix things. Maybe they could use their Weasley's Wizard Wheezes products in conjunction with time travel to make things right.
Voldemort would also be a key character. His obsession with immortality and his actions that led to many deaths in the series are fundamental to the theme of life and death. In a crossover, he could be pitted against new threats or forces that have different views on death, which would make for an interesting story.
Fred and George Weasley would definitely be the life of the party. They are always full of pranks and jokes, and at a house party, they would be the ones to liven up the atmosphere with their humorous antics.