Cheryl becomes personally involved in the case after the key witness in a federal money-laundering case and his wife are kidnapped and held hostage.
The 'Ex'-factor has to do with a co-worker Cheryl once dated and then, as we find out, was promoted over and also proposed to, since that was the solution to the FBI's rules about members of different levels of the command structure 'dating'. Married is OK, but please, no 'dating'. Anyway, the guy declined the marriage proposal. Couldn't handle the notion of being married to someone 'above' him in the hierarchy.
As Matt tells Emily when she wants to know what the deal is: "He's the reason Cheryl doesn't want us to date."
In other words, the episode wasn't about Cheryl, but about the flipside of the Matt-Emily thing; which made it 'personal'. I like the way these things are never just procedural, but always manage a personal subtext of some significance.
I also think that in due course Frank would have had the hots for Cheryl. You can see them setting it up. Pity it won't go anywhere, what with the series close to the end.
It also occurs to me that, after seeing Gina Torres in yet another series—the other was Firefly—it is clear to me that, while she has a strong presence, she isn't 'lead' material. Sorry, Gina Torres fans, but that's what it looks like to me. It's an apparent paradox, this thing about actors who stand out and yet somehow aren't 'lead' material. It just shows that the one doesn't necessarily follow from the other—just like other actors simply can't make the TV<->big-screen transition (either way). Funny thing, that.
In the 0-10 rating stakes, this one's somewhere in the 9.x.