Kinkade 'Kade' Sims is a consultant with the FBI.
He definitely is not an agent with the Bureau; rather, he is a man deemed of considerable use by Special Agent Morris, the man who approaches Sims with an offer for him to work on a case with them..
When we first encounter him he has been out of his desired career path, the military, and is working at Home Depot. In the months since his departure from the Service, "The beard he'd grown ... along with his scraggy shoulder-length black hair made him look about thirty when he was twenty-five. He'd added at least twenty pounds of bad weight on his six-foot-four frame". He is sharing an apartment with his childhood friend Alex and trying hard to maintain a low-key existence.
Prior to his current situation, as Agent Morris explained, "after your assignment at the National Counterterrorism Center (NCtC) and return to your home unit, you had a mental health referral and a hypomania diagnosis. Your top secret clearance was pulled, and you were then separated with an honorable discharge." According to the recap that Morris gives, Sims had been well thought of in his work with the NCTC. "Senior officers who worked with you at NCTC say you did an excellent job and your ethics were never in question. Your peers loved working with you and you're greatly missed. You just seemed to run into recurring problems off duty."
It was the pronouncement of 'hypomania' that got him deemed no longer someone the military wanted part of their ranks. Sims is in disagreement. "I still dispute my discharge from the army for that diagnosis. The physician I'm seeing reviewed my records and thinks my separation was a real stretch. It's a mild case, and very manageable. But the army never gave me a chance to prove it." Whatever Sims believes, does not alter the facts - he was out of the military.
Back to the FBI's approaching him for help, there was a particular talent that he had shown that the Bureau wanted and needed. It was shown when a month after starting to work at the NCTC he was invited to take part in a DARPA study where that defense agency. DARPA was testing a new, state-of-the-art lie detector and Sims was one of the guinea pigs. In the test, every single answer Sims gave resulted in "false positives, meaning the device said [he] lied on every question, even though [he] only lied on the questions he was told to lie on". Medically, the analysis produced the belief that the part of his brain that is active when a person was lying was, for Sims, always active, even if he was telling the truth.
For someone working undercover, that has to be a useful asset.