Daiya was the older brother to Mondo, who founded and lead one of the most feared biker gangs in Japan, known as the "Crazy Diamonds." Mondo considered Daiya his only family growing up, and was the only person who he could trust and respect. Mondo wanted to measure up to his older brother, and so he would imitate him in everything he did.
When Mondo thought about how he would have to take over the gang from his brother someday, he became determined to become stronger than Daiya, and prove himself to the gang's other members who didn't believe that he could take Daiya's place.
On the night of Daiya's retirement ceremony, Mondo challenged him to a street race. During the race, Mondo became reckless and sped into the wrong side of the road, and in the way of an oncoming vehicle. Just before he was struck, Daiya pushed him out of the way, taking the blow for himself. He died in Mondo's arms, and told him not to let the gang be broken up. It was a "promise between men," which Mondo did his best to keep.
During the Killing School Life, the threat of exposing that he had inadvertently caused the death of his brother was used to motivate Mondo into committing murder. When his secret was finally revealed, Mondo expressed immense guilt for not only killing his brother, but for not being able to keep the promise he made to him.
- “Ever since I was a kid, there was one thing my brother told me over and over again... He said that no matter what, a real man ALWAYS keeps his promises. That's what he left me.”
- — Mondo talking about how his brother told him a real man always keeps his promises.
- “M-My bad, kid... I fucked up. Sorry. Hey, kid... The rest is up to you. No matter what, you gotta keep the gang together. Cuz it's the team...you and me put together. It's...a pr... ...a promise...between men...”
- — Daiya as he lay dying in his younger brother's arms.
Many fans were moved by the tragic story of the Owada brothers, with some even citing it as one of the saddest moments in the entire series. While there is only a limited amount of information given about the relationship between Daiya and Mondo, fans believe the two were close and cared for each other dearly. Most fans prefer it as just a familyship, as opposed to an incest relationship.
- When put in the order of older brother, and then younger brother, Daiya and Mondo's names mean "Diamond." This is connected to the name of their motorcycle gang, the "Crazy Diamonds."
- The name of the gang is a reference to the character Josuke Higashikata from JoJo's Bizarre Adventure, whose stand name is "Crazy Diamond."