Ichigo Kurosaki is short-tempered young teenager of Japanese descent with an unusual hair color, that is bright orange, who most of the time has a near-permanent scowl on his face. Although he does not know it, he has an impressive lineage that he has inherited from his parents, which gave him the ability to be able to see spirits for as long as he could remember.
Until his 15th birthday he was only a regular human that could see and touch spirits without any special abilities, then one day, he met a Soul Reaper called Rukia Kuchiki who told him about the afterlife, evil spirits called hollows who eat souls, and Soul Reapers who purify them.
Shortly after their meeting, Rukia was seriously injured by an hollow attack and was forced to transfer her soul reaper powers to Ichigo in order to save both of their lives and the lives of the Kurosaki family, transforming Ichigo into a substitute soul reaper, while she herself lost most of her powers.
Due to the strict laws of Soul Society, the place where most of the dead end up after their death or through konso, Rukia Kuchiki was declared as a wanted criminal due to the transfer of soul reaper powers to a regular human, and her brother, the captain of the 6th division of Gotei 13 Byakuya Kuchiki and his lieutenant Renji Abarai were sent to the world of the living to bring her back to Soul Society in order to face punishment for her actions.
Ichigo Kurosaki tried to defend Rukia Kuchiki out of his personal sense of duty to her for saving his family, opposing the laws of Soul Society, but was defeated by Byakuya Kuchiki after winning against Renji Abarai, having the soul reaper powers he got from Rukia removed and being left to die out in the rain.
Kisuke Urahara decided to save him for reasons he did not reveal and promised to restore his soul reaper powers and open a way to Soul Society so Ichigo could rescue Rukia.
As the first step in his plan, Ichigo's spiritual energy was restored by Ichigo having to fight against a little girl named Ururu in his soul form while still having a complete soul chain, as a persons spiritual energy was known to increase when the soul is in danger.
Ichigo completed the first task set by Kisuke and began the final part of awakening his soul reaper powers right away, known as the shattered shaft, a theoretical way of awakening a soul reapers powers, developed by Kisuke Urahara that has never been done before, in which his soul chain would be severed and he will be forced into hole of despair in which he has a time limit of around 72 hours in order to awaken his own soul reaper powers or he will turn into a hollow and be killed.
Ichigo has already been in the the hole for over 72 hours and has not been able to awaken his own soul reaper powers despite his many attempts to escape from the hole and now his final chain of fate is about to completely disappear, while he himself is screaming while undergoing the hollowfication process by having a hollow mask start to form on his face, with his body still being intact.
"I have to help him." says Ururu, trying to save Ichigo from becoming a hollow, only to be stopped by Kisuke.
"Stop." replies Kisuke "There is still a slight chance that he will become a soul reaper," while thinking about the irregular transformation process "Just wait and see.", while silently praying 'please don't become a hollow, Ichigo. I do not wish to tell Isshin that his only son has died today.'
As the transformation process is in the final stages, Kisuke finally orders his assistant Tessai to casts his kido on him in order to destroy Ichigo before he turns into a hollow, starting with bakudo, while lowering his hat and having a sad look in his eyes.
Suddenly there is an explosion of high spiritual energy, which easily destroys Tessai's high-level kido, followed by intense flash of light that momentarily blinds all of them for few seconds, and Ichigo's spiritual energy completely vanishes, becoming something which they could not sense.
As all of them regain their vision, they see before their eyes an unknown soul that does not give out spiritual energy at all and does not appear to look anything like Ichigo Kurosaki.
The person is an attractive young man in his early 20's, with shoulder-length silver hair and blue eyes, who looks like an foreigner, most likely from European descent by the looks of it, having a beauty that transcends that of regular mortals as his creator and father made him to be his most beautiful son out of all his creations, with it also being one of his reason for his fall from heaven. He is dressed in the exact same clothes as Ichigo when he entered the shattered shaft, only his spiritual clothes are now partially destroyed due to him being taller and more muscular than him, with despite being muscular his muscles being that of a swimmer, not a bodybuilder. By Kisuke's original estimate this man is most likely between 185 to 189 centimeters tall, which was not wrong as the man's exact height is 188 centimeters.
Kisuke was the first one to get over his shock after seeing this man, having already put the pieces together in his mind and having come to the realization that this person either is or was Ichigo Kurosaki, so he puts his hand on his Zanpakuto hidden in the cane, getting ready for combat should he be a hollow, while asking the person in front of him "Would you mind explaining who and what you are and what you are doing in my training ground?"
As Ichigo Kurosaki was in the process of completely transforming into a hollow, his soul woke up in a place without any light at all. Ichigo quickly realized that he must be dead now based on this place and this is what happens when a soul dies. He thought about how his family will react and what would happen with Rukia now that he is dead.
As he was deep in thought, he heard footsteps behind him despite this place being completely empty and not having any floor, so he turned around and found an attractive-looking young man who would be the same person who Kisuke would meet in the outside world.
"Well." said the man while looking at Ichigo with a look that is full of disgust "Out of all beings to be reincarnated into, why did it have to be a human.", speaking in Japanese that had a slight British accent to it.
"Who are you." asks Ichigo, while then then having a realization that is completely false "Are you my Zanpakuto?"
The man then starts to laugh "Hahahaha..., Father really was a bastard.", so he replies to him "No, I am not. My name is Satan Lucifer, and I am the devil, the leader of all devils and the fist of it's kind."
Ichigo was having a panic attack "Are you going to take me to hell with you then?"
Lucifer decides to have fun with him and replies "Yes.", being interested in how Ichigo would react to this.
Ichigo then has a look of acceptance on his face "I see.", being completely honest with him "I do not regret anything in my life."
Having had his fun, Lucifer gains a small amount of respect for Ichigo due to his answer "Despite not having lied to you, I was also not completely telling you the truth when I said you go to hell." so he decides to tell Ichigo the truth "The truth is that I am you. Or to be more accurate I am the previous reincarnation of our soul. When you became a soul reaper, I slowly regained my original memories when I was alive over time but not my physical form due to having access to a power source similar to that of demonic energy, which I am now certain should be a hollows spiritual energy, while an old man with brown hair wearing sunglasses and a woman with white hair dressed in white constantly argued with each other in a weird world that was filled with skyscrapers that was constantly experiencing snowstorms over who is your Zanpakuto spirit."
"When Byakuya Kuchiki removed your soul reaper powers you got from Rukia Kuchiki, both of the spirits vanished from this world which now has become an empty world. Most likely due to the fact that both your and Rukia's soul reaper powers were already awakened and you partially using them, meaning the moment your soul reaper powers were removed, you also lost both Rukia's and your own soul reaper powers permanently."
"So, are we both are going to be in this place forever?" asks Ichigo.
"That depends on you." replies Lucifer "We are currently in the process of becoming a hollow which I am currently trying to prevent, meaning we have some time as the time flow in side this place is much slower than in the outside world. We can leave this place, but only if our souls merge together due to the hollow energy and my demonic energy being compatible. However doing so, both of our original personalities will cease to exist, with my personality most likely becoming the dominant one due to my soul being stronger than yours, with us becoming a new entity and a devil once we return to our physical body as our body will try to adapt to our soul which would be that of a devil."
"If I cease to exist, then as a final wish, promise me you will save Rukia Kuchiki from execution. " replies Ichigo.
"Alright, I will promise you that, as it does not seem to be an impossible request. However as I said before, our souls will merge together, meaning even if you had not made this promise, there is a chance that our new soul will not only have both our memories, but also our personality could be more like yours."
"So?" asks Lucifer, while offering a deal "Do you accept our soul to merging into one and us becoming a devil. If you are not honest and do not wish for this merging to happen, there is a great chance our souls will be destroyed and we would cease to exist forever. Although we are different personalities, we are still the same soul."
"Yes." was Ichigo's reply after thinking it through for a few minutes, even if he will not be Ichigo Kurosaki anymore, at least his soul would not be destroyed and he got to keep his promise to save Rukia.
Lucifer's then covered his hand in red demonic energy and pierces through Ichigo's heart, just like Rukia Kuchiki did with her Zanpakuto when she originally made Ichigo into a soul reaper, while the world that was previously empty exploded in a bright light similar to that of a big bang.
Authors Notes:
The world of DxD and Bleach are completely different universes, but Lucifer will eventually return to the world of DxD with the help of the Hogyoku.
Lucifer will not get a Zanpakuto, and will likely recreate some hollow abilities or kido with his demonic energy while not gaining any actual hollow abilities.
Lucifer's personality will change as a result of his and Ichigo's personalities or the pieces of souls merging together, adopting many key personality traits from Ichigo Kurosaki and becoming a much better person in general, with one of them being that he would care about those close to him due to both his previous memories and personality influencing him.
Once Lucifer's and Ichigo's newly merged soul enters Ichigo's original body, his physical body will start to change over time, with him later becoming a pure-blooded devil and him getting a completely new appearance, I have not decided how long it would take for this to happen (days? weeks? months? !not Years).
I am unsure about Yhwach and may make Aizen the main villain of the Bleach arc.
Demonic energy differs from spiritual energy. As soul reapers, hollows and souls are the supernatural elements of the Bleach world, the new Lucifer will be able to also see them, however due to demonic energy not being spiritual energy, he will only appear as a regular human to others unless they use a special device that can measure demonic energy which Kisuke or Mayuri could make sometime in the future.
Should someone wish to adopt this story they can contact me as I am not certain of my abilities. I will at least try to make the second chapter for this story.