Denji's Father

Personality
Little is known of Denji's Father's personality, as he was deceased prior to the chronological beginning of the plot.

Makima describes him as an "abusive alcoholic", who was violent enough with Denji to the point he was killed in self-defense.

Denji's Father is also implied to have an addictive personality, losing large amounts of money to the Yakuza resulting in a massive debt which Denji later inherited.

Control Devil arc
Makima, revealing her intentions to Denji, reveals that Denji's Father died not from suicide, but rather from being killed at Denji's hands in an act of self-defense. His supposed suicide was, in-reality, a ruse created by the Yakuza in order to have Denji inherit his debts.

Denji had been repressing this fact behind a psychological barrier, and when the memory comes to light Denji sees himself as a child, standing over his father's wounded and bleeding corpse.