When Byakuya saw Makoto was sleeping, he stuck out of his room and searched for Monokuma.
"Monokuma, I need you to come out and talk." Monokuma didn't listen.
"Well, then I guess I have to go to option 2 if Monokuma won't cooperate." He went back to the hallway where everyone slept, but instead of going back to his room, he rang the doorbell of Junko Enoshima's room.
"Ughhhh, what are you doing here so late?" Junko asked him, sleepily.
"I need to know about your sister. Mukuro Ikusaba. And if she's the 16th student." Now you've most likely beat the game or watched the anime, but if you haven't I recommend you never read a fanfiction about a series you haven't finished, also to go finish the game or anime. (I recommend the game) How did Byakuya know that Junko was the mastermind? Well, let's take a trip down Togami's memory lane to two days ago, when he was talking with Kyoko. He went up to her door and rang her doorbell.
"Byakuya?" Kyoko said.
"I know the truth now. I know that whoever your sister is is the mastermind of the game. You are the 16th student."
"Sister? I don't have a sister. At least I don't think I do. No, I definitely don't have a sister. Why do you think I'm related to the mastermind though?"
"I bet you can't talk with the cameras watching you. Come on Kirigiri, we're going to the bathhouse." And so, Togami dragged Kirigiri, without her permission to the bathhouse so they could be in private.
"Tell me everything you know about the outside world. And who the 16th student is."
"...Oh so that's what you met. Well, I'm sorry, I don't have a sister, and I don't know anything about the outside world but..." I bet you know what Kyoko's gonna say next.
"Mukuro Ikusaba. The sixteenth student, lying hidden somewhere in this school. The one they call the Ultimate Despair. Watch out for her." Then she walked away.
"GOD DAMNIT KIRIGIRI YOU ALREADY WHISPERED THAT IN MY EAR FOUR TIMES ALREADY! IN FACT, YOU SAID THAT IN EVERYONE'S EAR FOUR TIMES!" But all jokes aside, this didn't get him anywhere to who the mastermind is. If what the letter he got was true, then the mastermind and sixteenth student were twin sisters. He also knew the 16th student was with them, playing the killing game since she knew him and Makoto were in a relationship. He had a hard time believing it was Celeste, Fukawa, or Maizono since the Mukuro said she loved the despair. Those people looked miserable when they confessed to being together. So Mukuro had to be pretending to be someone else and he guessed it was Kyoko. But it had to be someone different. If it wasn't Kyoko, why has she just not tell Byakuya about her true identity? So he had to find a girl who has a sister, is in love with Makoto, and is obsessed with despair. He could eliminate all the boys. Himself, Makoto, Mondo, Leon, Kiyotaka, Yashiro, Chihiro, and Hifumi. Now he could eliminate all the girls that he knew didn't have a sister, this was easy because he made Makoto tell him everything he knew about the other girls, which was a lot. Toko, Celeste, Kirigiri, Sayaka, and Sakura. Then the last part. This took Makoto a bit to remember, but he told Byakuya about that time after Monokuma had just told everyone they slept together and Junko acted super crazy. Now he didn't know if Junko had a sister but she seemed to be the only one who matched all the other criteria.
So now there he was, with Junko who had a surprised looking face when Byakuya confronted her about it.
"I have no idea what you're talking about. You're being super weird." She gave a small signal to the camera and he knew what this meant.
"Then we're going to the bathhouse to discuss this." He dragged Junko to the bathhouse and sat her down.
Nobody commented on the last part where I was asking for help so I decided to make a different Izuru x reader fic. I also have two other fanfictions I'm writing after I complete this one. (I think only two parts left) Also I'm really sorry if I messed up somewhere when I explained how Byakuya knew about Mukuro, even I was a little confused about it.