Well, the various races in DxD would have diverse reactions. The dragons in DxD, like Great Red, might sense Ichigo's power and either choose to ignore him or keep an eye on him depending on how they perceive his threat level. The devils in high - ranking families would be worried about how Ichigo's presence could disrupt their political and power games. Meanwhile, the more innocent characters in DxD, like Asia, would probably befriend Ichigo quickly, seeing his good heart and his willingness to protect others despite his intimidating powers.
The characters in DxD would be quite shocked at first. Ichigo's appearance and his powers are so different from what they are used to. For example, Rias Gremory would probably be intrigued by Ichigo. She might try to form an alliance with him, seeing his potential as a powerful ally. Issei, being the main character in DxD, would likely be both jealous and in awe of Ichigo's abilities, and might challenge him at first but then learn from him over time.
He could fit in as a mysterious new student. His transcendent powers would set him apart from the rest. He might initially be seen as a threat by some of the factions in the school.
One possibility could be that Ichigo's unique powers as a Soul Reaper and his transcendental forms could create an interesting dynamic in the DxD universe. Maybe his Hollow powers would be seen as a new type of 'evil' power there, and the devils would either try to recruit him or fight him. Also, his Bankai and its evolutions could make him a formidable opponent against the various factions in DxD.
The story could then develop as Ichigo tries to find a way back home. Along the way, he gets caught up in the politics and battles of the DxD world. He might meet Issei and initially be at odds with him because of their different outlooks on power and combat. But as they face common enemies, they start to work together. Ichigo could also have run - ins with powerful devils like Sirzechs Lucifer, who might be intrigued by his power and want to either recruit him or eliminate him as a potential threat.
Ichigo and Issei would be really interesting. Ichigo, with his calm and collected nature most of the time, compared to Issei's more pervy and hot - headed ways. Their different personalities could lead to some really funny and intense interactions, especially when it comes to fighting side by side or arguing about battle strategies.
One possible plot could involve Ichigo's spiritual energy causing a ruckus in the DxD world. His power might be misinterpreted, and he has to prove that he's not a threat. At the same time, he could learn some unique abilities from the DxD characters, and they from him. For example, the devils in DxD might be interested in Ichigo's zanpakuto and its powers, while Ichigo could gain some knowledge about magic from them.
Well, it could be about Ichigo having some super - transcendent powers that he has to hide in the High School DxD world. Maybe he's a new student there and his powers start to disrupt the normal balance of the supernatural elements in the school.
Another aspect could be Ichigo's internal struggle in this new universe. He's used to the world of Hollows, Soul Reapers, and Quincy in Bleach. But in DxD, there are new moral and power dynamics. His sense of justice might be put to the test as he encounters situations that are different from what he's faced before. He might have to decide whether to interfere in the power struggles between the three main factions - devils, angels, and fallen angels - or stay out of it.
Korra would likely be very cautious at first. She's used to dealing with powerful opponents, but Hollow Ichigo's otherworldly nature would be something new. She might try to communicate with him, but if he showed aggression, she'd be ready to fight. Asami would probably be more strategic, trying to figure out his weaknesses. Mako and Bolin would be a bit more intimidated. Mako might use his lightning - bending as a first line of defense, while Bolin would be thinking of ways to use his earth - bending to either trap or slow down Hollow Ichigo.
Issei would probably be the first to want to fight Thanos. He's always eager to prove himself and protect his friends. He might initially underestimate Thanos due to his own growing power and confidence, but soon realize how powerful Thanos really is. Rias would be more cautious. She would likely try to gather information about Thanos first and then formulate a plan to either fight or negotiate with him.
Issei, being the hot - blooded character he is, would probably be excited to meet someone as powerful as Naruto. He might challenge Naruto to a friendly spar at first, hoping to learn from him and test his own strength. And as he gets to know Naruto better, he could start to see him as a brother - in - arms, especially if they face common enemies together.