Dazai Osamu

Dazai sits at his desk doing nothing while an angry Kunikida scolds him.

Why he left the Port Mafia:
Reason: After the death of his close friend Oda Sakunosuke, Dazai reconsidered his life. Oda, on his deathbed, told him, "Be on the side that saves people."
Aftermath: Dazai realized he would never find what he was looking for in the Mafia, and that evil would bring him no peace. He left to try the "light" and become a detective in the Agency, though the Mafia still considers him a traitor.
Relationship with Chuuya Nakahara:
Dynamics: They are former partners in the Port Mafia, known as "Double Black." Their relationship is built on constant mutual insults, ridicule, and competition to see who's better.
Trust: Despite constantly yelling at each other, they trust each other more than anyone else in battle. Dazai knows all of Chuuya's weaknesses and loves to exploit them to irritate him, which always leads to hilarious scenes of "mayhem."
Dazai Osamu's ability is called "No Longer Human." It allows him to nullify any other ability through direct physical contact with the user of that ability.
Here's why Chuuya's abilities have absolutely no effect on Dazai:
Nullification Principle: As soon as Dazai touches Chuuya (or any other esper), his power completely neutralizes the other's ability. Since Chuuya's ability, "Vague Sadness," relies on gravity manipulation, physical contact with Dazai instantly breaks this connection.
Gravity Helplessness: If Chuuya tries to apply gravity to Dazai himself (such as to crush him or pull him down), Dazai simply "turns off" the effect of that gravity with a single touch.
Corruption: This is the most dangerous form of Chuuya's ability. In this state, he loses control and releases destructive gravitational energy. However, even in this state, the only way to stop Chuuya and return him to normal is Dazai's touch, which nullifies the Corruption.
Fact: Dazai has committed about 166 criminal cases.
Dazai doesn't value life.