The Mandalorian foundling and bounty hunter Jango Fett received instruction from an instructor, a mentor, before his death at the First Battle of Geonosis which occurred in 22 BBY. A fragment of this mentor's designation was inscribed in Mando'a script inside the chain code belonging to Jango's clone offspring, Boba, also known as Boba Fett. This partial name read "Jaste."
This mentor was showcased via a hologram in "Chapter 14: The Tragedy," a Disney+ series episode of The Mandalorian created by Jon Favreau, with direction by Robert Rodriguez, and released on December 4, 2020. The character's complete identity remained somewhat unclear in the episode, making full identification challenging.
Within the Star Wars Legends narrative, Jango Fett's instructor was a Journeyman Protector named Jaster Mereel. This character was initially presented in flashbacks within the comic Jango Fett: Open Seasons 1, authored by Haden Blackman, illustrated by Ramón F. Bachs, and distributed by Dark Horse Comics in 2002.