A mild-mannered therapist and researcher, Dr. Takuto Maruki uses his research into "cognitive psience" and his training to try and ease the mental pain and suffering of everyone he can. Outside of his counseling sessions, Maruki seems clumsy, unkempt, and prone to small accidents. Despite this, he earns the respect of most of the students and faculty at Shujin, including the Protagonist's party, being a respectful listener and helping the school deal with the aftermath of the Kamoshida incident.
However, Maruki's calm and inviting demeanor hides a grief-stricken history that drives him to warp the entire world with his desire to save everyone, leading him to become the final antagonist of Persona 5 Royal.
In the past, Maruki's fiance, Rumi, suffered a tragedy that left her catatonic and barely able to speak after witnessing her entire family murdered by criminals. Maruki tried to think of a way to cure her, and unconsciously ended up making a deal with Azathoth (though he didn't know its name at the time) to take away her pain. Maruki miraculously saved Rumi, but at the cost of rewriting her memories so that she completely forgot about him and her family.
With this breakthrough in mind, Maruki planned to expand his research into cognitive psience. At first, his University was willing to grant him money and space for his research, but later he was shut down with little explanation other than a "lack of evidence" to continue support. Despite the setback, he continued researching cognitive psience on his own, using powers granted from his Persona to open a clinic and start working on real people.
This ended up bringing him into contact with a girl named Sumire Yoshizawa, a girl dealing with Survivor's Guilt who was depressed and having suicidal thoughts after the death of her twin sister, Kasumi. Using his powers, he changed Sumire's cognition to think of herself as her twin sister, Kasumi. Though most people still saw her as Sumire and played along out of respect for her grief (or fear that she could relapse into suicidal thoughts), the Protagonist and others close to him were affected by the change in cognition and saw her as Kasumi.
Later that year, Maruki saw Mementos merging with the real world outside as he visited his old Professor. Maruki was able to come into direct contact with his Persona, Azathoth. Using Azathoth's power and understanding the wishes that the Phantom Thieves had revealed to him during his one-on-one counseling sessions, he began changing reality with his own, planning to create his ideal world free of suffering and pain.
However, his plans are thwarted when the Protagonist, Goro Akechi, and Kasumi all realize something is amiss with the new world. Soon after, they free the other Phantom Thieves from their respective illusions and fight back against Maruki before he can finish overwriting reality.
Maruki challenges the Phantom Thieves with his own Persona, Azathoth, and then evolves it into Adam Kadmon. When the Phantom Thieves defeat him again, Maruki tries merging himself directly with Adam Kadmon in a last-ditch effort to win, but the Phantom Thieves come together as Joker shoots Adam Kadmon in a weak spot, shattering Maruki's plan. As the Metaverse starts to disintegrate, a wounded Maruki fights a wounded Joker in a one-on-one fistfight eventually conceding defeat.
Maruki disappears after the final battle, but is reported to have taken a day job in the real world, with his heart changed by the Phantom Thieves. As the Protagonist is about to go back home, Maruki appears driving a taxi and takes him to the train station, free of charge.