Spencer Reid
Oc/Reader is Female. (She is like Wynonna From the TV show Wynonna Earp)
She is Arrested By the BAU. They think she is a serial killer, And is holding her half sister hostage.
They think she had snapped after years in the army. (You can choose different, but she has to be able to fight and uses guns)
She is Interrogated By Everyone but she won't talk, Until Hotch says something about her half sister and she snaps, Lunging For Hotch( She had picked the cuffs) and Slamming him on the table and she yells at him to never speak of her sister again.
The team rushes in Guns drawn, And get her to let Hotch go.
Garcia comes running and says the actual serial killer is on the phone, She says they said that They have 10 Hours before babygirl is dead.
That enraged Oc/Reader as she Yells that she isn't the serial killer and if they don't let her go her little sister is dead.
The actual serial killer was A woman from the same unit as her and she became obsessed with Oc/Reader, and the reason why she Kidnapped her little sister was because she knows she is everything to Oc/Reader, But she wants to be everything to her So she is going to kill her sister to replace her.
After they save Oc/Reader little sister, and she kills the unsub, Hotch is impressed with her and gets her to join the BAU.