Fan Casting Unstable Horror Ally in Sort by Tropes

In a horror story with a singular protagonist rather than a Dwindling Party, it's very hard to find help. The creators want to keep the audience on their toes, so if the protagonist has an ally, that ally can't be too nice or comforting. In order to maintain a sense of tension, ally characters in horror works tend to have some scary traits, despite not being the main source of horror in the story. They may have creepy appearances or voices. The eeriness of this character will often cause the audience to question whether or not they are actually trustworthy. In order to be a straight example of this trope, the character must be trustworthy after all.