Only black character is a criminal? Yawn ...

I have been watching this show since the very beginning, so I know Avery had reminded Nelson (the black guy) about his criminal past numerous times in the beginning...mainly to warn him not to commit anymore crimes in the future. And rightfully so, Nelson was a former black hat hacker who committed numerous crimes during his time as a hacker, and was eventually caught...he escaped any prison time only because Avery decided to recruit him for her "hack-for-good" program at the FBI, which is a program aimed to turn the lives of criminal hackers round, by having them use their hacking skills for good. In a way it's a program for criminal hackers to seek redemption for their past crimes, by using their skills for good. If it wasn't for Avery, Nelson would've ended up in prison for his past crimes, but since he joined Avery's program he has redeemed himself and is using his skills & knowledge for good, and it was only possible because Avery gave him that chance.
And no, Nelson is not the only criminal on the show, the female Asian analyst on the team Raven was also a criminal, like Nelson, she was also a former black hat hacker who committed numerous crimes in her past. By the time Nelson joined the team in the beginning of the show, Raven had been working on Avery's team for 2 years already and she too has redeemed herself.
In that perspective Avery has saved them both and helped changed their lives for the better...there are also other former criminal hackers who Avery has recruited into her program over the years and saved...and one of those other former hackers that we know of is white (who you will see in episode 5)...but he turned out to be a traitor and is currently in prison.

So no, Nelson is definitely not the only criminal on the show, and it's not a matter of race, among the 3 former criminals who were recruited by Avery over the years that we know of so far, 1 is a black (Nelson), 1 Asian (Raven), and the other is a caucasian, so it's quite diverse.