When the strange pressure descended on all realms, the first thing Shunsui did after delegating all of his duties was to open the Muken. As he never returned the key for it to Central 46 after the Great War, he could enter the place anytime. Naturally, some members of the Central 46 tried to take it back but after threatening them with revealing Ichigo's involvement with the war and his Quincy lineage to the public, they all stopped.
Shunsui truly wanted to recognize Ichigo's effort for saving the Soul Society and the whole world at large, but he couldn't do so since the recovering Soul Society was still not prepared to accept a Quincy as their savior. In any case, it was a very useful card to use whenever he wanted to contradict Central 46's decisions, although he won't be able to use it many times.
After he entered the Muken, he went straight to one of the 'Greatest Wall' of Soul Society and if the owner of that pressure intended to take on the Seireitei, he'd be coming for Aizen first. Without any hesitation, Shunsui used his Bankai on him and pulled him into a game of 'hide and seek' with Aizen as the 'it'.
Because of his Bankai's ability, only Aizen will be able to find him. In that state of 'hiding' however, Shunsui won't be able to do anything aside from hide unless he cancels his ability.
For the whole week, Shunsui is hiding near the Central Great Underground Prison with an amused Aizen searching for him to stave off his boredom.
And as a side effect of that game, Shunsui was able to monitor the whole 1st Division barracks and prison without anyone able to perceive his presence. The Beasts of Hell were rampaging in multiple places within the Human World throughout the whole week and he thought that no one would be coming after the strange pressure disappeared, yet here he is, facing an unusual guest, 'Hiding for a bit longer was a correct decision.'
Shunsui prepared himself to attack at any given moment but when he confirmed that the strange man and woman were not doing anything strange, he also kept his hands, "Who are you? An envoy of Hell perhaps?"
Since Kisuke is wearing a Shinigami garb, Shunsui assumed that he came out of Hell. And just like the Beasts of Hell and Szayelaporro, neither of them exudes any form of Reiatsu aside from that of a normal Human would. And since he came from Hell, Shunsui predicted that he was at least on Captain level and the fact that he was able to silently enter Muken using a strange technique makes him very dangerous.
'We couldn't perform the Spirit Festival Ritual for Yoruichi and Kisuke because they disappeared instead of dying with their bodies disintegrated like the Old Yama so he or she is probably neither of them. A Shinigami of another generation?'
Kisuke, meanwhile, thought about his question, 'Looks like the problem with the doors of Hell is only escalating. I have to confirm if there's an intention of invasion from the other side soon.'
Everyone became quiet, trying to gauge each other, except for one lady, "Boss, isn't this fine?"
Kisuke looked at her with interest, "What are you trying to say?"
"It's not you to hesitate like this."
Kisuke became even more confused at her words, "Hesitate? What do you mean?"
Meanwhile, the other two just listened closely to their conversation without interrupting them.
"Just drag them to your problem. It's not like they can refuse. In any case, you need more people to understand or at least, know what you're doing so that you don't have to create all sorts of workarounds just to accommodate a few things."
Kisuke thought that Flanna was just misunderstanding things but then his mind went back to the time before they entered this world.
The Soul King is trying to do something but most of it was failed attempts and his return was also one of these attempts. He doesn't know what he's trying to do but if he's really the one who urged Byakko to send him and Yoruichi away, he might be trying to show him something.
But despite thinking more about it, as he just didn't have any information, that's all he could glean from it and he wasn't even sure that's what the Soul King intended. Then again, he had a slight idea of what it is.
What Flanna was telling him now was to drag these two to the unknown 'problem' that the Soul King presented him that might catch up to him without him knowing, '...And is it a coincidence that the doors of Hell pried open from the other side or is it an eventuality?'
Kisuke then looked at Aizen and Shunsui and thought that these two people are actually the most trustworthy. Aizen is a power-hungry guy that wants to destroy whatever is limiting him. Meanwhile, Shunsui is very different from the stubborn and strict former Captain Commander as he's someone who would disregard tradition if it means it is something he has to do. They are trustworthy because if given enough reason, they could work side by side without the others knowing.
Kisuke sighed and came to a decision. Even without saying a word, Flanna knew that she was able to successfully urge him into ditching his 'one-man army' mentality, "I'm not needed here anymore so I'm going back first."
Since she couldn't just go in and out of existence like Ophis and Lilith do whenever they're together, Flanna first had to open a hole to leave.
This shocked both Aizen and Shunsui because this is the Muken where the concentration of Reishi is the same as the Soul Palace in addition to multiple security measures that blocked any spatial manipulation within it, so cutting open the space would need a special procedure and equipment. However, Flanna just did it like she was just going out for a walk.
"Alright, before we start, I have a question for the both of you." Kisuke grabbed their attention back.
"I have a feeling that you'll be offering something big, go on." Aizen's interest and curiosity about the pair deepened.
Shunsui sealed back his Zanpakuto and said, "Whatever it is, I'm not letting you out of this prison." Of course, he's talking to Aizen. The strange man probably had the same ability to leave abruptly as the strange woman from earlier so there's no point in stopping him. What he needs to do is stop him from freeing the other prisoners down here.
"That's a shame." Aizen chuckled before turning his eyes back to Kisuke.
Kisuke went quiet for a moment before asking his question, "Do you think this world is a mistake?"