(This goes along with @redjade_22 and my AU of Monty x Mariah.)
The two didn't get along at first. It took Mariah getting shot during a break in during the day of a crazed parent who no longer had custody of the child. The police would come, detain the parent, and tell her to go get her wound treated. She would of course start walking to a First-Aid station, but would pass out right infront of Monty's room. Right where he could see her. Mariah could wake up to Monty wrapping her abdominal region. And telling her
"Oh, I thought I lost you. You weren't breathing, or moving, or anything." And Mariah would halt him from bandaging any further and explain everything.
"I'm... an Afton child. I was born around fifty-five years ago. In Nineteen Seventy One. The reason I look so young and that I wasn't breathing is because... I'm not human. Nor am I alive."
And Mariah, would go on to explain the curse that is brought upon EVERY second eldest in every Afton family. And that she got the curse. She then explained her death.
"It was May first. Nineteen Eighty Six. I was only fifteen. A school shooting happened and the attacker was pointing a gun at Vanessa. I wasn't thinking and JUST as the trigger was pulled... I jumped in the way. Effectively saving Nessy, but getting myself killed in the process. I've TRIED to let my soul rest. To be freed. But nothing has worked. So I'm stuck."
Then, Mariah pulled off her jacket to explain the next thing. Her left arm. There was a VERY visible line on the shoulder. She explained everything about Freddy Fazbears Pizza Place. And... her irrational fear of speakers that she has. That Monty made fun of. How a speaker tore her left arm off in the fire and that now... she was terrified of them.
(Sorry for this being so long, it's a scenario/headcannon.)