Chuuya's Relationships with the rest of the ADA
Kunikida is obviously his partner. Despite how well they work together and understand each other, they also drive each other up the wall. Kunikida is driven mad by Chuuya's knee jerk reaction to jump into a fight or run off to investigate when he "has a feeling something is wrong," messing up his schedule and dragging him into trouble. Chuuya is driven mad by how Kunikida "has a stick up his ass" and sticks to his Ideals even if it is actively shooting them both in the foot (metaphorically). Despite that, Kunikida does respect how Chuuya's gut instinct is always correct somehow and Chuuya does respect Kunikida's ideals (at least, most of them).
Yosano is one of his best friends. They very frequently get together after work to drink wine and shoot the shit. Early on, Chuuya and Yosano struck a deal where Yosano would practice various torture and medical practices on Chuuya; as a result, Yosano gets more knowledge and Chuuya develops a far higher pain tolerance than almost anyone in even the Port Mafia. This may have slightly skewed Yosano's baseline as to what a normal person's pain tolerance is... a fact which is the frequent topic of her and Chuuya's gossip sessions.
Kenji is Chuuya's favorite. The kid absolutely adores him, and as a result he has special privileges. He's the first (and only) person who hasn't been initially afraid of Chuuya, and Kenji still thinks that Chuuya is the coolest person in the ADA. Because of this, if he needs anything, Chuuya is dropping whatever he is doing to help him. In fact, he treasures the "Coolest Big Brother" mug that Kenji gave him, and it sits on his desk alongside the other "cool city trinkets" that Kenji finds and gives him.
The Tanizakis are a little bit afraid of Chuuya. When it comes down to it, they trust him to have their backs, but he is their elder and member of the ADA's Shittenou. Naomi doesn't cross paths with Chuuya often, but whenever Junichirou is on a case with Chuuya, his nerves get the best of him. Chuuya knows he makes the younger detective nervous and gives him his space.
Gin was Chuuya's favorite before Kenji joined the agency. She actually briefly met Chuuya before he joined the agency, and when they reunited on a case Chuuya was working on, he invited her to take the entrance exam and join him. Since she doesn't have an ability, she's just a clerk; the alter ego who learned to use a knife under Chuuya with a peculiar ability is definitely not her. As Chuuya's first mentee, she got spoiled with him giving her his coat and paying for her uniform. She is absolutely not jealous of Kenji.
Atsushi admires Chuuya a lot. After all, Chuuya is the one who brought him into the agency, showed him some of the ropes and even taught him how to fight and control his ability. While he's obviously learning from everyone in the agency, he gravitates towards Chuuya (which drives Kunikida up the wall), and Chuuya tries his best to be a good influence on Atsushi. Although 'good' is up to interpretation, considering that there have been multiple occasions where Chuuya has given Atsushi alcohol despite being underage (and had Kunikida ream him out for it).
Kyouka is terrified of Chuuya at first. Her first impression of him was Chuuya, full of rage, using his ability on the entire cargo ship to sink it and threatening to kill everyone on board. Learning of his past in the Port Mafia and even before as a 'Portent of Death' and 'The Kamikaze of Yokohama' did not help... but what it did was give her hope. After swallowing down her nerves and talking to Chuuya, she finds that he's actually really nice, even inviting her to talk to him about anything that's on her mind. While still mostly hanging out with Atsushi, she does take him up on that offer many times, and Chuuya absolutely did not cry when she and Atsushi gave him flowers as a thank you gift for being a good mentor.
Ranpo sees Chuuya as one of the few people that can match his intellect and stand on equal footing with him. For the longest time, they were rivals; Ranpo initially heard of this child genius who hung around the police station and "was as smart or maybe smarter than Fukuzawa's boy" and could not let that stand. Even after Chuuya joins the agency they constantly butted heads, with Chuuya viewing Ranpo as an annoying dickhead and Ranpo viewing Chuuya as a hotheaded and stubborn fool. Until one case where, without even needing to coordinate, Chuuya completely understood everything going on and made a plan that complemented Ranpo's perfectly. Since then, Ranpo has acknowledged Chuuya as an equal and they now mostly get along, with their competitions being friendly fun as opposed to an ego contest.
Fukuzawa is Chuuya's boss, and the person he respects most in the whole agency. While Chuuya does keep his secrets, his loyalty to Fukuzawa is as absolute as it was to Mori; if Fukuzawa asked him to jump off a bridge Chuuya would do it without question. It's unsettling to Fukuzawa at first when Chuuya got on one knee to pledge his loyalty and when he saw how dogmatically that loyalty is, but he starts to slowly understand Chuuya over cups of tea and over years of working for the agency. He starts getting glimpses into him through the strays that Chuuya brought in, and starts making progress into getting him to let down the walls and reveal who he is behind them. To Fukuzawa, it doesn't matter that his ability somehow doesn't work on Chuuya, he considers Chuuya one of his own anyways.
Mine and @breadstickcat's OCs under the cut:
