Which are the real characters in The Shakespeare Stealer by Gary Blackwood?

It is easier to start with which characters are only fictional and were not real individuals connected with the authentic Globe Theatre. Based upon current research, Widge himself, "Julian" Cogan and Simon Bass are the three who were not real individuals. The sharers, the hired men, the manager, the trainers, the caregivers and the major apprentices ("prentices") were authentic people who fulfilled functions in the Lord Chamberlain's Men at the Globe Theatre. Even "the boy named Nick" may have been Nicholas Tooley, a player in the Lord Chamberlain's company who was noted for specializing throughout his career in playing female roles. Blackwood's Nick puts up a fuss over continuing to play women's parts but finds himself back with the youthful apprentices when he fails to learn and correctly execute his first male role:

Before [Nick's] second week was out, the part in Love's Labour's was given to Chris Beeston, and Nick was back upstairs with [the prentices], practicing his swordsmanship.

Even Dr. Timothy Bright and his system for "shorte, swifte, and secrete writing" were authentic, he being the "chief physician of St. Bartholomew’s Hospital" in London and his system having been published as Characterie in 1588 (Frances Henderson). The names of those in The Shakespeare Stealer who were authentically involved with the Lord Chamberlain's Men along with William Shakespeare at the Globe Theatre include these:

Robert Armin: son of a tailor, played the fool; one of Widge's trainers.
Christopher Beeston: entrepreneur and actor.
Richard Burbage: financial backer for the Globe Theatre; friend of "Mr. Shakespeare," both dislike "others pawing them" in "the tiring-room."
"Henry" who dropped his lines to whom Beeston refers may be Henry Condell, an actor.
Alexander Cooke: a tragedian (specialist in tragedy); known in this story as "Sander," Widge's trusted friend.
Richard, who commented on the stage makeup of Julian, Sander and Widge ("Widge, a little less whitewash"), may have been Richard Cowley, an actor.
William Kempe: noted singer and dancer in comedies; known to Widge as "Will Kempe" who "left the company" under a cloud of suspicion regarding As You Like It and Leicester. 
Thomas Pope: actor, musician, dancer; he "caught the dirty dastard," Widge, then gave him a home with "regular meals and soft bedding."
William Sly: an actor; "a prentice like [Widge] a few years ago ... [now] a hired man."
Augustine Phillips: actor, dancer; a "small, athletic-looking man."
John Heminges: partial financial backer; speaking with a stutter, he welcomed Widge into the "Lord Chamberlain's company."
https://www.britannica.com/topic/Globe-Theatre

https://www.shakespearesglobe.com/learn/teaching-resources/