Reader/Oc is female.

She is an amazing at reading body language, reading micro expressions, hand to hand combat, and she is an incredibly talented marksman

She has lots of tattoos. (tributes to victims, dot for every person she killed you can choose different if you want)

She was a Criminal. She hunted and killed Serial Killers, paedophile, Rapists.

She went all over the world tracking them and then killing them.

She has saved lots of victims of Serial Killers law enforcement didn't know existed.

She had been doing this for over 5 years (you can choose different if you want)

But she got caught after staying to Give a victim Medial Attention.

She is put into a supermax prison. (you can choose different)

When she is interviewed she reveals all the Serial Killers etc she has killed.

And where she stashed all the information she gathered about them with Proof of their Crimes.

She tells them where she buried their bodies. She has killed almost 100 serial killer, Rapists, Paedophile etc

The government makes a deal with her she works with the BAU and they reduce her sentance. But she has to obey lots of rules.

The Team isn't on board with her joining them. Especially Garcia but after saving their lives multiple times the start to accept her.

Garcia likes her after she sends her pictures of cute things everyone they get a case.
She listens to Spencer's facts and actually enjoys them.
She helps JJ with her profiling and combat.
She bonds with Derek with their sense of humour and hatred of child abusers.
She bonds with Emily for being the outsider
(you can choose different if you want)

Spencer gets a crush on her.

And after a while Oc/Reader relises she cares for Spencer differently from the others. (she has never been in love)

They start dating after a while