The nature of Bankoku-gai as a city could be extended to Suo as a character.
When the gang first arrived in the city, everyone (sans Nirei who had already researched about it, so he knew what to expect), was easily distracted by all the restaurants and attractions. Especially Sakura, who’s whisked away by how boisterous the outer circle of Bankoku-gai is, in turn detracting himself from their initial goal. He nearly falls for its façade of prosperity.
As for Sakura falling for and getting carried away by charades…
Sakura’s initial suspicion of Suo was bordering on curiosity, even. But after Suo had elected him as class captain and done so much to help Sakura succeed, Sakura was lulled into a false sense of security. He’s one of Suo’s closest friends; of course he knows everything about him! There's no need to question Suo’s nature further despite Sakura’s former doubts.
In a similar manner, Bankoku-gai’s exterior is very flashy. Packed with a multitude of restaurants, stores, and eye-catching decor whilst being surrounded by a river, it’s a great location to stop at if you’re a tourist. After all, its diversity offers so many new things to explore. And Kasai’s geographical nature as a river island only isolates it from the other districts and even parts of mainland Japan.
If you don’t actually probe deeper into the city, it seems like a very quaint area. The outer circle of Bankoku-gai is only 1.5 kilometers long, so its dazzling nature is merely skin deep. In a way, it’s almost illusory, lulling unsuspecting outsiders into a false sense of security even.
Isn’t this also reflective of how Suo acts on the surface? He’s described as always joking around with his friends (Sakura in particular being the butt of it). His fighting style could be considered flashy too; a rather foreign one considering that it’s aikido, an art that focuses on parrying attacks rather than direct hits (much unlike the way most of the Furin kids fight). Suo is incredibly intelligent too, with his insight reaping benefits for the parties involved. He is also a very hospitable and kind person to others. Much like how Bankoku-gai most likely relies on tourism for its economy, Suo lets his outward disposition shield his true vulnerabilities.
As for the inside of Bankoku-gai, every entrance is locked. And heavily guarded. Its true nature is barricaded from the rest of the world, despite the steel doors being plain as day between the multitude of colorful stores. Much like how Suo keeps his friends at a distance, offering them advice and letting them open up to him, yet not reciprocating that vulnerability.
It also compounds Nirei’s written information. He’s incredibly good at investigation, so he’s able to find and compile lots of information on a topic he’s interested in. Though Nirei researched the outer circle of Bankoku-gai well, he wasn’t able to find much about its inner circle due to how protected it is. Similar to how Nirei didn’t have much information on Suo’s feats, yet he’s able to accurately pinpoint one of Suo’s weaknesses (to which the latter didn’t take kindly).
When Sakura and co. do try to access the residential areas, they’re rejected. Multiple times. They don’t have the passes needed to access the inner circle, so Sakura has to resort to asking for Suo. And the further they try, the more frustrated Sakura gets, because there is a “no pass, no exception” policy instilled for Bankoku-gai. But he’s desperate to reach Suo and won’t give up despite this massive barrier.
So Sakura, alongside Nirei, Sugishita, and his upperclassmen try to breach the residential again in the most convoluted way possible: by deception. Since directly asking won’t work, they’ve resorted to underhanded tactics (thanks to Endo). And it worked!
When Sakura does observe the surroundings of the ‘real’ Bankoku-gai, he attributes it to Suo. He doesn’t even call it ‘Bankoku-gai’s inside’ or ‘The residential area’; he specifically calls it ‘Suo’s City’. A stark contrast to the beginning of Chapter 222, where Sakura does refer to the shiny outer circle as Bankoku-gai. Maybe he has an inkling about Suo’s true nature, given that Sakura witnessed his outburst in KEEL.
With its dilapidated buildings and worn, compacted infrastructure, the state of Bankoku-gai is… not very good. Perhaps it’s due to poor economic upturn or government corruption. Not to mention that this is the area where the Red Chanpurus holds its base. And to the non-Bankoku-gai residents and out of universe, the RC seems to be nothing but enigmatic and shady. If these rumors are even true in the first place.
But it’s also a place that Suo holds near and dear to his heart. He grew up and trained here. The people he cares about also live in this caged-off area, as well as the organization he works for. It’s fair to assume that Suo deeply loves inner Bankoku-gai and its people, despite all the problems it harbors. And he too wants to protect his hometown.
Though its run-down state could also reflect how complex and haphazard Suo’s inner self is. His own turmoil, adversaries, worries and the “things people are better off not knowing about” are all enshrined in this place. Away from the rest of the world. It also reflects the lack of introspective internal monologue Suo has despite being a pretty important deuteragonist.
All of this is guarded by steel gates that don’t let outsiders in. Because neither the Red Chanpurus nor Suo himself want any external persons to know of their issues, because they can handle it themselves. Their strength comes from the deep, pre-existing bonds they share after all. But despite their extravagant and modest exterior, they’re deteriorating from the inside.