Optimus Prime would likely be a very calm and noble presence among the RWBY characters. He might be seen as a sort of mentor figure, especially to Ruby, who is also a leader in her own right. His stoic nature could contrast with the more emotional and impulsive behavior of some of the RWBY characters.
Optimus Prime could fit in as a powerful ally. He would be like a super - powered being in the RWBY world. His presence could shake up the power balance. Maybe he arrives through a portal and immediately starts helping the RWBY teams fight the Grimm. He could be seen as a symbol of hope, much like the Huntsmen and Huntresses are in RWBY.
Optimus Prime's strength and ability to transform could be combined with RWBY's unique semblances. For example, Ruby's speed semblance could be enhanced when she rides on Optimus in his vehicle form. Or Yang's strength semblance could be amplified when she teams up with Optimus in a fight, using his power as a boost.
One idea could be that Optimus Prime somehow ends up in the RWBY universe and teams up with Team RWBY to fight a common enemy. Maybe a new type of Grimm that is attracted to his Cybertronian energy.
The other characters in the Highschool DxD world would have a wide range of reactions. The angels might see Optimus as an unknown factor and be a bit suspicious. The fallen angels might try to manipulate the situation to their advantage. And the normal human students would be terrified at first. But as the story unfolds and they see Optimus Prime's noble actions, they could start to look up to him as a hero, much like how the Autobots are seen as heroes in their own universe.
Some fanfictions show Optimus Prime as a father figure to Bumblebee. His wise and patient personality helps guide Bumblebee's growth. Bumblebee, in turn, with his loyalty and enthusiasm, gives Optimus hope and reminds him of the good they are fighting for. It's a very heartwarming interaction.
Well, a cool crossover in Optimus Prime x RWBY fanfiction could be the Autobots' technology being integrated into the RWBY universe. For example, Optimus might help Ruby upgrade her Crescent Rose with some Cybertronian tech. Also, the Autobots' code of honor could influence the huntsmen and huntresses in RWBY. Maybe there could be a story where Optimus and the RWBY team have to travel through different dimensions to stop an evil force that threatens both their worlds.
Writers often develop Optimus Prime as a wise and noble leader. They show his strategic thinking in battles and his moral compass when dealing with humans. For example, he might make decisions that protect both his Autobot comrades and the humans at the same time. Bumblebee, on the other hand, is usually portrayed as more youthful and energetic. His beeping and use of radio snippets to communicate add a unique charm. Writers might have him be more impulsive at times, especially when it comes to protecting his friends, whether they are Autobots or humans.
They could interact through combat. For example, the Autobots' powerful blasters and physical strength could be combined with RWBY characters' unique weapon - based combat styles. The Autobots could provide cover fire while the RWBY characters get in close for more precise strikes.
One way could be through some sort of multiverse concept. Maybe Optimus Prime's universe and the RWBY universe are connected by a rift, and the male reader, who has the essence of Optimus Prime, gets transported to the RWBY world. There, he could use his knowledge and abilities from Optimus Prime's world to interact with the RWBY characters.
At the beginning, they might be cautious around each other. Optimus Prime, being a more traditional Transformer, might find Optimus Primal's more organic and beast - like nature a bit strange. However, as they communicate, they would find common ground in their goals of protecting their people and planets. They could exchange stories about their battles and leadership experiences. For instance, Primal could tell Prime about how he led his Maximals in the Beast Wars, and Prime could share his tales of fighting the Decepticons. This exchange would help them build a bond and understand each other better.