Yusuke Meets the Ancestor Who Stole His Victory
SPOILER!
Raizen's blood is what revived Yusuke during Chapter Black, and Raizen is also the distant presence that took control of Yusuke's body against Sensui. Yusuke reaches Demon World furious about both.
Their first meeting is therefore not a warm reunion. Yusuke attacks Raizen and gets crushed. Only afterward does the relationship shift toward training, inheritance and the question of what Yusuke should do with a kingdom he never asked to inherit.
Why it matters
Yusuke goes to Demon World looking for the person who interfered with his fight, not looking for a father figure. That keeps the lineage reveal from erasing Yusuke's resentment or independence.
Debut: Ep 098 - Return to Demon World
Raizen Starves Because of a Promise to One Human Woman
SPOILER!
Raizen once entered the Human World, was treated by a human woman and fell in love with her. After their time together, he vowed that he would not eat another human until they met again.
She died. Raizen kept the promise anyway.
Centuries later, that decision is still killing him. His body is starving, his kingdom is weakening and the political balance of Demon World is collapsing around a private promise almost nobody alive remembers.
Why it matters
Raizen's story destroys the easy idea that Yusuke's demon inheritance makes him less human. The ancestor at the root of that inheritance changed his entire existence because of love for a human.
Debut: Ep 103 - Inheritance