Javier is the son of Leonora Cortes and Juan Rovira Cortes. He is a high school student living in La Palma who wants to be a mystery novelist when he grows up. Also known as the Locked Room Master (密室先生 Misshitsu sensei), Javier is the mastermind behind fifteen locked room murders.
Javier is not drawn in the novel's illustrations so his normal appearance is unknown. However, he disguises himself with makeup and clown clothes such as a red nose and a puffy wig when he invades the dreams of his victims.
Javier is an overconfident yet depressed teenager. He is sadistic as he enjoys psychologically torturing random strangers in their dreams and even purchases clown clothes and makeup to make their nightmares scarier. Despite aspiring to be a writer, Tsukumojuku Kato implies that Javier doesn't have what it takes, as when Tsukumojuku, George Joestar II, and the police had burst into Javier's house, they found notebooks filled with locked room tricks but no actual novels. He is also narcissistic as he would talk quite a bit about himself in the dreams of his victims, believing that they wouldn't remember anything when they woke up.
Javier has a Wound that allows him to travel through anyone's dreams.
Javier would comfort Leonora when his father, Juan Rovira Cortes would make her cry. Whenever Javier slept, he would be trapped in a locked room with his mother. Javier's desire to escape from his flesh while he was in that locked room with her manifested into letting him sleep and escape through dreams. At some point, Javier purchases a clown disguise to manipulate others.
Wearing his clown disguise, Javier enters the dreams of vulnerable victims, works his way into their hearts, and draws out their darkest emotions. He would first find someone his victim hated or had trouble getting along with, and then manipulate his victim to blame all their negative emotions on that person. He would twist their fears to make the victim think that person was the source of all their problems. Despite how forced the logic or reasoning was, Javier's target would believe him after his constant whispering to them in their dreams. He would go as far as to torture them if necessary, by either killing them in their dreams or having their enemy kill them. Javier's victims would awaken in a nervous frenzy and feel as if the real world was a dream. Eventually, Javier succeeds with fifteen victims and they murder their targets in locked rooms. As his victims do not remember the details of their dreams when they awaken, they assume they created the tricks for the locked room murders themselves.
He would get an unhealthy amount of sleep, as he was also sleeping during the day and entering the daydreams of his victims. Javier ends up being caught by the police when he attempts to target Penelope de la Rosa, who has a pathological fear of clowns. Penelope clearly remembers her dreams but nobody believes her aside from Tsukumojuku and George. They bring up the dreams to Javier's other victims and they end up remembering him. Javier had talked a lot about himself in their dreams, as he was overconfident that they wouldn't remember him. From their descriptions, George and Tsukumojuku were able to piece together his identity and draw a composite sketch. Javier admits to George that the one he really wanted to kill was himself.
Since they couldn't prove the ability to enter dreams in court, the Spanish police on La Palma beat Javier to death with their batons and sink his body in the bottom of the sea at night. George and Tsukumojuku were powerless to stop them and Leonora didn't even try. If stories spread about Leonora's son being a witch because of his mysterious powers, the church would end up being involved, and Leonora's entire family would be persecuted.