First of all, you switched from third person narrative to first on the second page or so, which is distracting in and of itself. Second: Jen is very much a Mary Sue.

She knows everything about the team? From Google? Maybe a few things, like Rossi's books and such, but not EVERYTHING about them. How would she know about Hotch's divorce? How would she know about Rossi's obsession with the old case? She wouldn't because there is no way that would come up in google.

Furthermore, I doubt she'd be allowed in the briefing at all, let alone go on the case with them. If you want to write about an incoming FBI agent, I suggest some research about the FBI and how becoming an agent works and such. As is, it's just not believable. I can see Reid being a "mentor" but, when I saw the word my first thought was a 'big brother-big sister' type of thing, not a 21 year old training to be an FBI agent.

She's just too perfect. As technically good (well written/presented) as your story is, it is far from believable.

but, while this is very nicely presented and well-written technically, Jen rings all the bells of a huge Mary Sue - from the trendy clothes and music to the dropping of cool anime references (in the original Japanese, of course) to the eidetic memory - and knowing things which, really, would she? How could she know that someone had resigned but the paperwork hadn't gone through until they'd reconsidered? Surely the whole point of the paperwork not having gone through is that nobody knew? And then there's the way she starts giving instructions.

Maybe I'm wrong. Maybe she's a canon character, and it's canon for the regular cast to take this sort of thing from kids who have basically no experience and are with them to learn. If not...you might want to reconsider her behaviour, her knowledge of them, and their reactions to her trying to take charge.