At first, Kazuichi and the Ultimate Yakuza seem to be at each other's throats quite often. Kazuichi makes comments towards Fuyuhiko such as "If you didn't have that big Yakuza family, you'd be nothing but a midget!" Which, of course, is returned by Fuyuhiko's own sharp tongue. As time passes, though, their disagreement with each other seems to dissipate, and they become tolerant of one another, and eventually open up to one another as they survived the Killing School Trip.
In Chapter 4 of Super Danganronpa 2: Goodbye Despair, they were sitting together on Monokuma's train.
In episode 2 of Despair Arc, they were playing a game together, along with Nagito Komaeda and Teruteru Hanamura. When everyone is under the effect of aphrodisiacs, Kazuichi sees this as his chance with Sonia. Fuyuhiko helps him control himself and zips up his zipper. In episode 6, Kazuichi points out to Chisa Yukizome that even Fuyuhiko, the Ultimate Yakuza who distances himself from everyone, decided to come to their party to welcome back their homeroom teacher.
In Nagito's world, Kazuichi, Fuyuhiko and Nagito were best friends, even going so far as to consider him his brother. After Kazuichi's death, Fuyuhiko wanted to avenge his death, trying to kill the murderer. Fuyuhiko even considered Kazuichi as a brother to him.
The ship sailed as a result of the bromance between the two, which developed with time as the boys got to survive the Killing School Trip together. Fuyuhiko trying to help Kazuichi control himself after being fed with the aphrodisiacs helped the ship's popularity. The bromance was even more prominent in Nagito's illusionary world, which fueled the ship even further.
- Soda K./Kuzuryu F. on FanFiction.Net
- Fuyuhiko Kuzuryu/Kazuichi Soda tag on AO3
- Fuyuhiko Kuzuryu & Kazuichi Soda tag on AO3
- Fuyuhiko's relationship with Kazuichi on Danganronpa Wiki
- Kazuichi's relationship with Fuyuhiko on Danganronpa Wiki
- Both were two out of five survivors of the Killing School Trip, and the only two male survivors aside from the protagonist.
- According to the series' creator, Kazutaka Kodaka, his favorite scene involving Fuyuhiko was when he locked in a grapple with Kazuichi under the effects of the aphrodisiac.
- Hiyokazuryu refers to the ship between the two and Hiyoko Saionji.
- Kuzusoumaeda refers to the ship between the two and Nagito Komaeda.
- Neapolitan Ice Cream refers to the ship between the two and Hajime Hinata.
- Strawberry Trio refers to the ship between the two and Gundham Tanaka.