Louis is talkative and chatty. He can be friendly, but his lines may contain the impression of unsettling and uncomfortable. He is described by Erice as a monster who is called from the land of the deceased and once again unleashed into the world.
Five years before the events of Fate/Requiem, a nine-year-old Erice Utsumi met Louis and his Master at a therapy session which Erice was forced to attend. Erice befriended the two of them and would visit the Master's workshop, where he made things like children's school bags and shoes. Aware that Louis's Master was somewhat mentally disturbed, Erice took it upon herself to help him. Erice thought that, by giving support and with Louis on his side, the Master would be able to live in the drastically changed society "After" the Holy Grail War and help him get used to and blend into the lifestyle.
Shortly after Erice befriended Louis and his Master, people began to go missing across Mosaic City without a trace, with their mutilated corpses found later in different locations. One day while visiting Louis's Master, Erice was told to no longer visit his workshop as he would be going somewhere far away, and let out a huge rant about the Holy Grail system and his absolute dissatisfaction about how the people's free will had been taken away due to the inclusion of Holy Grail and Servants into their lives. This was the last time Erice had a proper conversation with him.
Some days later Erice ran into Louis on the street and learned that his Master had not in fact left the city. She returned to his workshop to find her Dread Spirit abilities activating automatically. The further into the Master's workshop Erice went the more dead bodies she found, accompanied by the sound of Louis humming. Erice's memories after that are blank, but she awoke to find that she had killed Louis's Master in self defense.
Erice fell into Louis's Noble Phantasm La Grosse Tour, a recreation of his prison from life. Louis stood motionlessly and collapsed in her arms, fading away without his Master alive to supply magical energy. With his last words he called out for his mother.
Karin was one of Louis's kidnapped victims, and one of the few who survived. It was due to this that she and Erice first met. When Erice met Voyager five years later she was initially wary of him because he, like Louis, appeared to be only a young boy.
Marie Antoinette mentions Louis during the Orleans Singularity, saying how her son was killed just to hurt her. She calls him Charles, perhaps to differentiate him from his father Louis XVI; Louis XVII's full name was Louis-Charles.
Erice's encounter with Louis further haunted her when she met Marie, to the point that her own Dread Spirits manifested and took the form of Black Marie in response to her feelings toward Marie and Louis.