Alex was a young boy living with his family in the Murder House during the 1970s. A video projector shows him and his sister playing, and it seems Alex collected bugs. A story he drew, Child's Fable, describes his encounters with a monster that made him kill a "rhinoceros" and a "cat", presumably referring to his stuffed animals.
His mother, worried by this imaginary friend, once sent him away to camp, though he claims the monster came with him. This seems to suggest he suffered from schizophrenia, rather than the monster being one of the ghosts in the House or the Infantata. Though living in the House no doubt exacerbated the problem.
It would seem Alex eventually lost his mind, murdered his sister, and dismembered the body. The room has splatters of blood on several of the clues. An audio clue left behind by a man (presumably Alex as an adult) mentions that he "couldn't put her back together", and alludes to Alex having an unhealthy obsession with her.
It seems possible Alex may have also died in the House at some point, as the video projector shows a pair of ghostly hands reaching out to grab his sister. One of the clues in the 1970s Attic is a mummified creature, and the ghostly hand also makes bunny ears that are similar to the creature's actual ears, suggesting it is the monster Alex saw.