Warning: The following includes spoilers for The Night Beyond the Tricornered Window Episode 5, “Past,” now streaming on Crunchyroll.
Episode 4 of Tricornered Window basically confirmed that Mikado’s father is the mysterious “sensei” who is manipulating Erika to curse others for money. However, we don’t know his intentions or why he even left Mikado and his mom in the first place. This episode reveals his past -- and it's far more tragic than anyone would expect for such an evil antagonist.
As suspected, “sensei” is confirmed as Mikado’s father since they share the same voice actor. He meets Mikado’s mother, who is a civil servant. He claims that it's love at first sight, but it's also possible that he sees Mikado’s mother as some kind of protection spell that can help dispel the fearsome spirits around him.
According to him, the “air” around her is “clean and beautiful,” meaning that her positive energy cleanses the atmosphere and has the power to get rid of the negative spirits. On the other hand, he naturally attracts ghosts, and this probably puts a strain on him over time. This is the reason he is so drawn to Mikado’s mother, a bit like Hiyakawa is drawn to Mikado.
As we see throughout the series, most people who deal with ghosts don’t have a particularly positive outlook on life. Erika curses people for a living but is secretly tormented by what she sees and does. Hiyakawa doesn’t really have any social skills or a moral compass. Only Mukae deals with the ghosts with any sense of empathy and care, but it seems that his power is not as strong as others. Regardless, no matter how they deal with the spirits, all of them seem to be very fond of Mikado.
After Mikado’s parents marry, Mikado’s father seems to want to give up the life of a psychic. However, after Mikado is born, it soon becomes clear that he has inherited his father’s powers -- he can also see and attract ghosts but is also very afraid of them. Mikado’s mother’s positive energy isn't strong enough to hold off both powerful men, so she grows weak and has to go to the hospital. Realizing that his presence is eventually going to kill his wife, Mikado’s father decides to leave and remove all traces of himself.
It is obvious that Mikado’s father loved his mother, no matter what his motivations were at first. He says to Mikado that one of them had to leave so that she could be saved. He could have abandoned Mikado and erased all traces of Mikado's existence, but instead, he chose to disappear himself, leaving the biggest clue of his own existence behind. This means he also cares about Mikado and wants him to have a happy childhood with his loving mother. In a sense, Mikado caused his parents' separation, but it's also because of this that he gets to have a life at all.
This portrayal of Mikado’s father makes viewers question how he became the way he is now. We know he is using Erika for evil deeds, and that she fears him even more than she fears death, which she experiences on a daily basis due to her own horrific powers. He is cursing random places and people so that he can use the negative energy for his own purposes, whatever these may be. In addition, he is involved with the yakuza, so it's safe to assume he's no longer the loving family man from the flashbacks.
It could be that the pain of having to leave his family has driven him to a life of crime, or that without Mikado’s mother, he has been gradually taken over by the negative spirits around him and has finally given into the darkness. Either way, this episode shows that the Mikados, both mother and son, have the power to bring positivity to those consumed by darkness. In other words, it's up to Mikado to bring Hiyakawa back from the brink of villainy, which is exactly what he seems to be planning.
