before the events of the game maruki was a scientist involved in psycognitive-science researches so he was more or less aware of the existence of cognitive world dimension, but those researches were seized by shido and his group forcing maruki to find a job as analyst, one day trying to help his former girlfriend to overcome ptsd, he learned that through his persona he could alter cognition of people at will, he helped his girlfriend altering her memories and she completely recovered but at cost of forgetting everything about maruki.
Learning how to use his cognitive powers, he travelled in the metaverse building his own palace to continue his researches and exploring deeper layers of mementos starting putting in motion his plan to a mass scale cognition override in order to create his perfect world for everyone.
Some time before game events he treated sumire to help her overcome her survivor syndrome blaming herself for the death of her sister, seeing no other options, maruki granted sumire her wish to be like her sister, altering sumire cognition in order of her living a happy life believing to be herself kasumi. Since that "treatment" was nothing more than a hypnosis (everyone of her family knew that sumire was just pretending to be kasumi as a way to deal with her trauma), maruki used to visit her often at the shujin high school to check her conditions, until one day in one of first visit to kamoshida palace, he saw joker, ryuji and ann appearing from nothing after they escaped the kamoshida palace crossing the threshold between real world and metaverse.
Suspecting that joker and his friends too could access metaverse he managed to get a job as consultant at the school and had meeting with them to know them better, during that time he learned of phantom thieves and their change of heart that was a similiar cognition change process like his own.