Keiko and Senjumaru both started their life in the soul society, Senjumaru fully focused on completing her mission of making each squad more powerful decided to take the heavy load and train all the divisions Lieutenant and other members while leaving Captain's for Keiko to train. She also gave Keiko 6 months to improve everyone so she would punish him personally.
Keiko being both irritated and bored went to meet Yamamoto asking him to call all the captains.
Yamamoto seeing no harm did that, Keiko took out a chair for himself and seat next to Yamamoto in the meantime. Soon all remaining captains came together.
Seeing all of them here Keiko pointed at Byakuya and said: You over there tell me why are you so weak?
Byakuya and other captains looked around and made sure the finger was pointed at Byakuya, Byakuya raised his hand and pointed toward himself as if still confused.
Keiko annoyed screamed: Yes you, I'm the head of this clan and whatnot. Why are you so useless? You weren't even able to protect your sister.
Byakuya would have probably gotten angry at someone else but the person seating in front of him was both Older than him and more powerful than even the Captain-commander himself so he just lowered his head and listened.
Keiko pointed at Sui Feng next and asked: If I brake both of your legs without you running around what can that useless Shikai of your do?
Without waiting for anyone Keiko pointed at Zanpachi: You are so useless that you don't even know your Shikai's name.
Keiko acting frustrated stood up and walked toward the window of the assembly Hall and started laughing loudly and acting like a crazy person: hehehehehe yes yes why didn't I think this before. I will train all of you for 5 months after that if you're not as strong as I want I will simply kill you and blame it on harsh training or something. Hehehehehe Then I will be free from babysitting idiots again hehehehehe.
Keiko pointed at Yamamoto and moving toward all the captains said: Yes you all let's start with training. In this Phase I will try to kill you all you have to do is survive.
Keiko with a smile on his face simply disappeared from his position, Yamamoto screamed for everyone to draw out their Zanpakuto's and shield each other but just as Mayuri Kurotsuchi started moving Keiko appeared in front of him ripping out his head from his body and disappearing again and a low cold voice rang in everyone's ears: One down
What none of the captains including captain-commander realized was that Keiko had trapped them all in a Genjutsu using his Sharingan when he first started asking them questions and humiliating them.
Keiko wanted to let them build up a healthy fear of what a strong person can do to them and their loved ones before they begin working and begging for more power. Keiko let them continue getting buried deeper and deeper into Genjutsu.
In Byakuya's version, Keiko after ripping Mayuri's head appeared in front of everyone, blood from Mayuri's body showed on all of them painting their face and clothes red but as all of them were extremely skilled and experienced they didn't get distracted and understood Keiko was trying to kill them. Keiko disappeared again this time appeared in front of Zaraki who was standing in front of Byakuya, Keiko punched through the chest of Zaraki, passing his complete hand through his chest and took Byakuya's head in his hand and started pressing it. Byakuya felt extream pain of his skull being crushed when the doors of the assembly hall opened completely with Rukia and Renji along with other division 6 members rushing in, Byakuya at this moment forgot about the pain and started moving his hands to tell Rukia and others to run away but then he heard a low voice in his ear: My my your sister came herself to me. I should play with her a little bit too. don't you think so too little Byakuya?. Byakuya's eyes widened he tried to move by the hand on his face already disappeared. He looked around and saw, his fellow captain's swords broken and injured all over even his face disfigured a bit. He ignoring his fellow captain's plea for help and rushed toward Rukia who was running toward him. He saw that behind Rukia, Keiko appeared just behind members of his division 6 squad and ripped their head out. Using all his strength, Byakuya rushed toward Rukia but he saw himself moving extremely slowly along with Rukia, meanwhile, Keiko calmly walking next to each member and killing them one by one finally reaching Renji who was just behind Rukia, With a smile on his face Keiko looked at the terrified and disfigured face of Byakuya and gave him and ever charming smile before ripping out the head of Renji along with his spin from his body. Byakuya watched as Rukia looked back as blood fall all over her as she tried to scream but Keiko appearing right in front of her with a smile on his face completely covered in the blood slowly lifting Rukia in the air and giving another smile to Byakuya who using everything he had screamed to let her go before he watched Keiko break her neck in two. Byakuya broke down crying went straight to the floor on his knees.
Byakuya opened his eyes once again to see Rukia one last time before dying but found himself seated on the floor with everything alright with his body, and fellow captain's who appeared to be in similar condition as him completely baffled. He looked around and saw Keiko still seating in his chair giving his ever-charming smile. Byakuya took a deep breath to calm himself down before he heard the voice of the monster once again: Now shall we start again?
Keiko used the same tactic with each captain other than Mayuri who using his other brain believed himself to be semi-immortal. Keiko started twisting his mind making him believe that Keiko is slowly destroying his very soul thus erasing his very existence which finally worked like a charm with Mayuri. Different captain's believed and wanted different things thus using them became simple and efficient for Keiko.
Using Genjutsu, Keiko also lets them face their inter demon thus making them adopt to their Zanpakuto and inner nature more clearly. Knowing that the Bount arc will start soon in the human world, Keiko sends orders to his Arrancar to look out for the soul collecting bees which will be created using the only female Bount who can reproduce as sacrifices. Keiko was no saint, he was human and selfish he always thought that the MC of fanfiction who tried to fix everything and make everyone happy was very idiotic, power leads to conflict and conflict leads to chaos which then leads to pain suffering and destruction thus no matter how much one try society can't be fixed a balance can be created but it will be broken sooner or later if there is no end goal for the people to work toward.
Bounts were sad people with sad stories which Keiko was sad about but he was more interested in cutting them open and finding what makes and gives each doll it's unique ability then finds justice for them.
Keiko after torture..... I mean training them for 3 rounds let them out of the Genjutsu, with everyone on their knees Keiko smiled and laughing loud said: First round of training done for the day, I'm newly assigned assistant Advisor to improve and train all of you. What you all experienced today was something that is similar to Aizen's power. We will continue this for the next two days until I have crushed all your hopes and Wills. After that, we will begin the real training to make you a proper shinigami.
Keiko finishing his speech disappeared from his seat along with his seat.