Hey there, time to get patch with that whole rock situation. Now in canon Urokodaki told Tanjiro to open the rock because he thought it was going to be impossible and he didn't wanted to lose any more students, but I like the idea of opening that rock as the final task as a demon slayer, because it's like cutting the neck of one.
Also, I might have the timeline a bit off, remember Izuku got there a day after All Might broke his dreams, then he spent months with the Demon Slayer Corp, then first is the UA exam then it's the Final Selection. This is how its going to be. Now let's get to start.
A few months later
Izuku had been training nonstop with Masachika , mostly because the finals exams were in a few weeks . He couldn't believe that all that time has passed as he was near the two big exams of his life, he still remember when got dragged into the Demon Slayer corp and chose to be their intel on hero society.
Training with Masachika wasn't that easy, he was nicer than Shinazugawa but he was still a really capable and strong Demon Slayer. Izuku still had bruised from months ago that still haven't healed yet.
''Argh'' Said Izuku falling down on the ground and hard.
''Again!'' Said Masachika holding his sword.
Izuku flitch in pain while grabbing his sword, most of his tissue skin on the hands was gone and was all red and itching. He was really in pain.
''My hands hurt a lot! God! I can't keep up!'' Thought him.
''Come on kid, you want to be a hero than you have to remember that pain is also gain. Now come on do your best shot!'' Said Masachika.
''Yes!'' Said Izuku.
Izuku charged with his sword and even doing a breathing exercise on the way, he learned more from Masachika while the two were constantly sparing. Even though Izuku was still not able to achieve the total concentration he was getting near.
''Ha!'' Said Izuku trying to hit Masachika but he dodged.
''Slow.'' Said the older man.
''Hiya!'' Said Izuku with a swing that almost hit the older man in the face, but he managed to dodge it
''Much better!'' Said Masachika with a smile.
''Ha!'' Said Izuku trying once again, but Masachika escaped.
''Come on kid! You can't keep doing the same tricks over and over again. You need to use your head.'' Said Masachika.
Izuku grit his teeth with a lot of pressure, he need to use his brains and really do a really good job in hitting the older man.
He learned the ways of breathing technique from Masachika, but he still need a lot of concentration and contact with the air so he could do one of the attacks.
''My breathing still needs work, but I can still do a lot with just a sword.'' Thought him.
''don't get your head on the clouds! Your enemy won't be that generous.'' Said Masachika.
Izuku felt a foot coming to his face and the older man kicked him right in the jaw.
KICK.
''Argh!'' Said Izuku in pain as he hit the floor.
''Are you going to give up?'' Asked Masachika with a smirk.
Izuku got up immediately and soon prepared his sword.
''No, I am not going to give up!'' Scream him.
A few days later.
With time Izuku managed to memorize every single move that Masachika was able to do, the right angle, the right timing and the right projection. Today would be the day that he defeated him.
''You will be defeated today!"' Said Izuku all serious.
''After so many months you think this will be day?'' Asked Masachika with a smile.
''I will win for sure!'' Said him.
''You said that a few times but let's see what are you going to do.'' Said Masachika.
Masachika was quick and went to slap Izuku right in the hands to drop his sword.
Smack
However izuku predicted that and use the time to fully to a spin and hit the older man right in the chest.
''Uh!'' Said Masachika in shock.
''This is it! My only chance!'' Thought Izuku
Bam
The older man was then launched towards the ground, he was hit with that attack and that led a purple bruise in his skin.
''I did it!' I beat him! I can now cut the rock.'' Thought izuku in shock as Masachika just laughs.
''Ha Ha ha Ha.'' Said the old man laughing.
''Huh?'' Asked Izuku confused.
''Congratulation Izuku, after so long you truly show your potential as a demon slayer, well done and all in less than a year.'' Said Masachika.
''What do you mean?'' Asked Izuku.
''Your strength is now comparable to the members of the Corp that patrol the night. You sure has great potential in the future.'' Said Masachika.
''So can I cut the rock now?'' Asked Izuku.
''Kid, you already did that.'' Said Masachika with a smile.
''Uh?'' Asked Izuku confused.
The green haired teen looked to see the rock and after the mist left he could see that the rock was split in half. Izuku finally realized what this was all about, to get him to achieve his maximum strength that he cut the rock without realizing it.
''I did it! I really did it! Masachika-san thank you so much.''' Said Izuku
However the older man wasn't there, he disappeared and Izuku had no idea where he could be.
''Masachika-san! Where are you?'' Asked Izuku.
While Izuku was looking for the man that helping him out one of the crows who was watching all this went flying to report to the headquarters about this achievement.
At the Ubuyashiki mansion.
The crow landed on Kagaya bedroom and did a little noise to inform that he has arrived.
''Oh hello there, what do I own the pleasure?'' Asked Kagaya.
''Izuku Midoriya has cut the rock in half.'' Said the crow.
The leader of the Demon Slayer corp just smile at that, in the end his plan was fully working.
''So in the end he did it. In less than a year, he are truly blessed.'' Thought him.
Later
Sanemi heard the report and went to the grounds to see if Izuku did in fact cut the rock, upon arriving he found the huge bolder cut in half and he was inspecting to see if there was any sort of tricks that were used to do it.
''You cut it clean.'' Said Sanemi giving his verdict
''Thank you sensei.'' Said Izuku.
''Come with me.'' Said the Hashira.
''Right.' Said Izuku
At the Ubuyashiki Mansion.
The marked man was gathered with the hashiras plus Kanae to congratulated Izuku on his accomplishment, he was now truly worth of doing the final selection and have strength enough to take on the UA exam.
''Congratulation Izuku, I knew you could have done a great job. In the end my prayers were answered.'' Said Kagaya.
''Thank you master.'' Said Izuku bowing.
''With this Sanemi has given you authorization to do the final selection and I will have all your documents signed to be the one sponsoring you at UA. Congratulations my child, you are one step closer to achieve your dream.'' Said him.
Izuku just started to cry. After so long he was finally going to do it, he was going to be the first Quirkless Hero in Japan and he had the Demon Slayers to thank and also for give him a new perspective in life.
''Rest for 3 days and we will send you to your mom. We know that the least you want is to spent the next days before the two big exams with her.'' Said Ubuyashiki.
''Thank you.'' Said Izuku.
'I got say I am impressed that you did such a good job on your own Izuku-kun.'' Said Shinobu.
''Yes, you did a really good job in there! I can tell you have now a true swordsmanship hand.'' Said Rengoku seeing those bandages on Izuku hand.
''Oh it was brutal, I almost fainted from all that pain, but I knew I had to keep up, and if I am going to take out demons I need it to show them that I am not afraid.'' Said Izuku.
''The rock is something that very few manage to cut in half. Its pretty much like the neck of a demon, trust me they will be a lot tougher to kill.''' Said Gyomei.
''Yes, I heard that but luckily, I wasn't alone to deal with this whole training.'' Said Izuku.
Sanemi was now confused upon hearing that.
''Who was there helping you? We gave order for none to go there.'' Said Sanemi confused.
''Oh Masachika-san helped me.'' Said Izuku/
The Hashira looked at him in shock and confusion, only Kagaya was having a little smile on his face.
''What did you say?'' Asked Sanemi now pissed.
''I say that Masachika-san helped me, do you know where he is at sensei? I would like to…'' Said Izuku with a smile but then Sanemi grabbed him by the shirt.
''What are you saying! Were you that near that you saw him! Don't play games with me kid!'' Scream Sanemi.
''Uh?'' Said Izuku confused.
''Tell the truth! Who helped you! There is no way…'' Said Sanemi still pissed and ready to yell a lot at Izuku until Ubuyashiki stopped.
''Sanemi, that's enough.'' Said Kagaya all calm.
The wind Hashira immediately dropped Izuku and bowed down to apologize for this behavior.
''My apologies master.'' Said him.
Izuku was confused as he gets his shirt fixed.
''Master, what is going on? Where is Masachika-san? I want to thank him for helping me.'' Said Izuku.
''Izuku, do you believe in spirits and in ghosts?'' Asked Ubuyashiki.
''Uh yes…?'' Asked Izuku a bit confused.
''My child, Masachika Kumeno has been dead for years.'' Said the leader/
That revelation shocked Izuku, if Masachika was dead then who was the person helping him.
''What! No, he told me that he was a member of the Corp and he knew Sanemi-sensei.'' Said Izuku.
''Did he say that he knew the ``Wind Breathing Style``?'' Asked Kagaya.
''Yes.'' Said Izuku.
The cursed man knew what that meant as he looked up.
''I guess spirits of fallen child has come to help you.'' Said him.
''Oh my.'' Said Izuku.
''So the spirits have come to our aid even on our hardest moment, truly the world is a mysterious place.'' Said Gymoei.
''Why does this feel a little weird.'' Thought Giyuu.
Sanemi still didn't like this, he hasn't heard the name Masachika in years and he still had horrible memories of that day.
''Please I want to see him! I want to thank for all lessons he gave to me.'' Said Izuku.
Ubuyashiki just smile as he knew Izuku has a good hearth.
''Very well Sanemi, Kanae please escort Izuku to Masachika.'' Said him.
''Yes master.'' Said the two.
Izuku was accompanied by the hashira and ex hashira to see where the man that helped him cut the rock was at. He was still not sure what was going on but Ubuyashiki knew.
''Looks like even the other side wants to help you achieve your dreams.'' Thought him.
At the cemetery.
Izuku saw a huge amount of tombstones and graves. It was like the biggest cemetery he has ever seen in his life.
''Are these…'' Asked him but Sanemi answers him directly.
''Fallen Demon Slayers, all who gave up their life's to protect the world. You don't see their names in a museum of heroes.'' Said Sanemi.
Izuku just kept looking and looking, it looked like there were miles and miles of tombstones. It was a bit horrifying.
''My god.'' Said Izuku.
''It's sad, but they all fought to protect the world. The master always make sure to notify their families and make sure they have enough money to survive for the rest of their life's.'' Said Kanae.
Upon walking for more 5 minutes the group soon arrived at their point.
''Here it is.'' Said Sanemi.
Izuku saw a tombstone with a name on it.
Masachika Kumeno
He just got on his knees and look in confusion at that, there was even a picture of him and Izuku couldn't believe at that.
''But I saw him! He was standing right next to me when we were training.'' Said Izuku.
''You must have seen his spirit. Maybe he wanted to help you out, you don't need a quirk to see ghost Izuku.'' Said Kanae.
''I…'' Said Izuku in shock.
''He was my superior when I started in here. I wanted to do nothing with this place, but he showed me the way and how to fully work the ''Wind Breathing''', I would probably just be a homeless punk if he hadn't showed me the ways of the Corp. He died before I became a hashira, but we managed to kill the demon bastard who killed him.'' Said Sanemi.
Izuku couldn't believe that. So a spirit helped him cut the rock in half, what a truly shocking moment this was. He couldn't stop crying and he didn't know if he had the strength to pull off those attacks or not.
''I am really that fit to be a demon slayer?'' Asked Izuku knowing wondering if all that training was just a hallucination.
''You did well Izuku-kun, you are now in the right path of becoming a great hero and a great Demon Slayer. We are all putting our fates on you.'' Said Kanae.
''Get up kid, we are still train before you go home.'' Said Sanemi.
As Izuku did a silent prayer to Masachika and thanked him for all the help he gave to him during these months of training. Truly Japan hasn't changed their ways even after the quirk era.
Later
Kagaya was on his room finishing to write all the documents and permission to announce that he was sponsoring Izuku for the UA upcoming year. His wife was helping him writing the documents while some Kakushi members were waiting for the next step.
''And done, please send these documents to the UA website and to Mister Principal.'' Said him.
''Yes master.'' Said the Kakushi.
Lady Nagant was also present, just wondering what else would Ubuyashiki would like to ask.
''Master, you know they will try to hunt you down. Even when I was a pro I heard of Mr. U.'' Said her.
''Yes, I know but we managed to hide from demons, and we have some of the best hackers and trackers that Japan has to offer. We will be in a safe place my child.'' Said Ubuyashiki.
''Agreed, there is no way a hero can find us in here. Besides, I don't want them to spoil things in this paradise.'' Said Nagant.
''Of course, Izuku is aiming to be a real pro that isn't addicted to money or victories. That the pure definition of a hero.'' Said him.
''I just hope the kid don't think too much about aiming for the top. That's like a drug and so many get addicted.'' Said her.
''I see, if something happens, we will correct him. I want you to be with the Kakushi in case a hero news happen, and we have to be on the lookout for.'' Said him.
'Yes master.'' Said her.
Ubuyashiki ssmile as he saw that everything was working on it.
''Are you ok dear?'' Asked Amane.
''UA exames are in a few weeks and the Final Selection after, Izuku has a lot to show to the world, but I have faith in him and what he will bring to the world.'' Said him.
He looked at the widow wishing he could see the sky and hoping to be talking at Muzan direction.
''This new era will the one that finally eradicated him. Muzan, your days of life are finally going to end!'' Thoguht him.
The head of the Demon Slayer Corp knew that killing Muzan was a hard task. But he knew that this era of Hashira and Demon Slayers were special, and they would be the ones to end the tyranny and horror that the Demon King has started 1000 years ago.