Describe the characters of Newjack: Guarding Sing Sing.

Your question is about the characters in Ted Conover's Newjack: Guarding Sing Sing. Newjack is the true story of Conover's time at Sing Sing prison, where he was a guard.
Conover himself is the primary character; the experience of being a guard at Sing Sing is told from his point of view. Conover is an experienced journalist with a history of involving himself in situations in order to write about them from an insider's perspective. He decided to train as a prison guard and work at Sing Sing when he was unable to get journalistic access to the prison.
Conover is transformed by his time as a prison guard. He says:

Prison got into your skin, or under it. If you stayed long enough, some of it probably seeped into your soul.

Conover seems humbled by the experience, especially when he sees that you can't tell what a person's character is from their appearance. Some people he expects to be in prison for white-collar type crimes are actually there for violent crimes, for example. 
Conover narrates with precision, showcasing his meticulous nature. He wants the reader to see the world he lived in for almost a year. His attention to detail and careful reflection are strong characteristics. He says, describing keys from the prison:

The pewter-colored cell key was the biggest, its shaft as thick as a Mont Blanc pen . . .

Conover changes throughout the narrative and questions his own goodness when he feels an unwilling thrill as an inmate is beaten.
Other prominent characters include the following:
Mama Cradle, the overweight correctional officer. Cradle is a tough person but has the best of intentions for the staff and prisoners alike. Cradle is forced to balance the needs of the inmates and the corrections officers along with the demands of the state.
Officer Smith, a fellow correctional officer who Conover admires. He sees Smith as a person who's able to see the humanity in prisoners and act accordingly. A major theme throughout the book is how prisoners are dehumanized, and Smith is someone who showcases the best of what correctional officers can aspire to be.
Dieter, an ex-marine who Conover rooms with during training. Conover says of him that they were fundamentally different. Dieter used substances, disliked women and liberals, and woke up early, whereas Conover had opposite habits and beliefs. 
Toussaint is a gang member who is questioned by the COs of Sing Sing after a physical incident. His reflections on gang membership cause Conover to reflect on the nature of gangs, the conditions that cause them to exist, and the people in them.
Sergeant Wickersham is an abusive man with a troubled past. He was once held hostage during a prison riot, which may help explain his violent disposition.
Most of the characters in Newjack are described with a few personal details that reflect their place in the prison system. They are both affected by and described by their jobs and positions at Sing Sing. Conover's character is the one most in focus because, of course, the book is based on his own experience and written from his perspective.
https://archive.nytimes.com/www.nytimes.com/books/00/05/14/reviews/000514.14bergnet.html?mcubz=0