I think Weiss would be very important. Her family's influence and her own skills in combat and magic could play a big role. She could use her resources to bring in extra help or supplies to strengthen Beacon. Also, her strategic mind would be valuable in planning defenses.
It might be Ozpin. He has a vast amount of knowledge and experience. In this au, he could use his wisdom to guide the students more effectively. He might have some secret plans or knowledge that he reveals at the right time to ensure Beacon's safety.
Ruby could be the most important. Her leadership skills and determination are crucial. She has the ability to inspire others, and in this au, she might be the one who first senses the impending danger to Beacon and rallies everyone to take action.
One possible plot could be that the characters are able to prevent the fall through better communication and teamwork. They might discover the enemy's plans earlier and work together to strengthen the defenses of Beacon. For example, Ruby could use her speed to quickly spread warnings, and Weiss could use her glyphs to fortify key areas.
Blake could also be a great character to focus on. Given her own complex backstory and her tendency to be more introspective, she might have deeper insights into the characters and situations in the Undertale AUs. She could form unique relationships with some of the more morally ambiguous characters in the Undertale multiverse.
Ruby Rose is often a very popular character. Her innocent yet determined nature makes her a great focal point. Fans love to write about her adventures and her growth at Beacon Academy. Another popular one is Weiss Schnee. Her complex family background and her own journey of breaking free from the expectations of her family are interesting to explore in fanfiction.
I think Weiss might be the most powerful. Given her intelligence and magical abilities in RWBY, she could easily understand the complex mechanics of Pokemon training and battles. Also, she could potentially use her Glyphs in creative ways to enhance her Pokemon's performance in battles.
Captain America would be very important. He has always been the moral compass of the Avengers. His leadership skills and ability to bring the team together would be crucial in a reunion. He can bridge the gaps between different members, whether it's Iron Man's tech - based thinking or Thor's more otherworldly approach.
Ruby would be really interesting. Her speed and enthusiasm would make her stand out in the For Honor battles. She'd be eager to learn all the new combat moves and would probably be the first to charge into a fight, her scythe whirling in the For Honor arenas.
I think Yang would be great. Her fiery personality and powerful punches would be a force to be reckoned with in the Battlefield setting. She could be the one charging into enemy lines, taking out multiple foes at once, and really showing off her combat skills in this new and different kind of battlefield.
It's hard to say for sure, but Ruby Rose has the potential to be very powerful in a crossover. Her silver - eyed warrior powers are still somewhat of a mystery in RWBY, and if combined with the knowledge and abilities from Hellsing, she could become a force to be reckoned with. Also, her leadership skills and the fact that she is constantly growing in power throughout RWBY could give her an edge in a crossover scenario where she has to adapt to new threats like the vampires from Hellsing.
I think Ruby and Violet would be really interesting. Ruby is all about speed and combat, while Violet is more reserved and has the power of invisibility and force fields. Their different personalities and powers could create some really cool interactions. Ruby could learn about being more stealthy from Violet, and Violet could learn about being more assertive in combat from Ruby.
I think Naruto and Weiss would have some great interactions. Weiss is from a high - class family and is very proper, while Naruto is this rough - around - the - edges ninja. Their clash of cultures could lead to some really funny and also some really deep moments. Maybe Weiss could teach Naruto about the importance of elegance in combat, and Naruto could teach Weiss about the value of hard work and determination from his own experiences growing up.