The mentor's name was encoded within Boba Fett's chain code.
The mentor was mentioned on a hologram in "Chapter 14: The Tragedy" of Jon Favreau's Disney+ series The Mandalorian, directed by Robert Rodriguez and released on December 4, 2020. The character's name was partially obscured within the episode, which made it difficult to determine the individual's full name.
In the Star Wars Legends continuity, Jango Fett's mentor was Journeyman Protector Jaster Mereel, who first appeared in flashbacks in the comic Jango Fett: Open Seasons 1, written by Haden Blackman, penciled by Ramón F. Bachs, and published by Dark Horse Comics in 2002.