Shinichiro was the first family Izana had after being orphaned and loved him dearly. He exploded at him when he found out they weren't actually real brothers; which lead to him feeling extremely lonely again. He later took over Shin's gang.
Izana was jealous of Mikey of how Shin gave him affection also, wanting to be sole brother to Shin when he found out that he was actually not related to Shin his jealously strengthened. Ultimately wanting to destroy Mikey.
Emma and Izana lived together until Izana was brought to the orphanage, promising they'll be together again. But ultimately not. Izana was the one to order the death of Emma. In his last moments, he said Emma's name as hints of his regret about what he did and still love her as his sister.
Izana and Inupi met when they were kids at Shinichiro's bike shop. Inupi served under Izana as his right hand man during the eight generation of the Black Dragons. Inupi later disliked Izana for the way we changed the gang Shin worked hard for.
Kakucho and Izana met when they were both in the orphanage. Izana was as much a leader as a child as he was as an adult; telling Kakucho to follow him and eventually create their own kingdom in which they named 'Tenjiku'. Founded by Izana the gang was strong, Kakucho as his second hand man; he sought to destroy his younger brother. Kakucho is the voice of good reason for Izana and keeping him grounded, as he was going mad. They strived for a utopia together. Planned from childhood that they'd never leave each others sides. When Kisaki begun firing at Kakucho, Izana's body moved in auto pilot jumping infront ulimately killing him. In his dying moments, he smiled telling Kakucho that he was the only one he has, and as Kakucho cried, hoping he would die to be together again they held hands as they did as children. In the end, Kakucho lived, ultimately becoming the ending their era.