Dr. León Castillo is a character in the Fran Bow universe. He is only known by his diary that Fran is given by the toad in the Second Chapter and his appearance in a photo, found in the final chapter of the game. This photo reveals that he worked at the Oswald Asylum at some point and was perhaps even a leading member of the team researching twins. The Fran from the Ultrareality also mentions him having come and visited her and explaining things to her about the ultrareality. Among these things is that you must be broken to enter the ultrareality.
Aside from the photo in chapter five, the only physical representation of León we get throughout the game is the self- portrait from his journal.
In an excerpt from the journal, he refers to himself as "one of the thousand." From this, we can infer that he is one of a thousand people (including Fran) that has access to the other realities of the FREE.