Tobias Beckett is a professional thief played by Woody Harrelson in Solo: A Star Wars Story (2018). StarWars.com describes him as "a native of Glee Anselm" who "has planned and carried out any number of heists with his crew", always "aware of the angles and uncertainties — and ready to unholster his blasters if something goes wrong".
The mentor slot, occupied badly
Structurally he is Han's Obi-Wan, and the film knows it. He recruits the young Han Solo on Mimban, teaches him to shoot, teaches him to plan, and teaches him — repeatedly, verbally, at every opportunity — that everyone will eventually betray him. Han takes the lessons and does not take the warning, which is the film's actual subject.
His crew
He works with Val, his partner, and Rio Durant, an Ardennian pilot. The Vandor coaxium job kills both of them inside twenty minutes of screen time, which is what puts him in debt to Dryden Vos and Crimson Dawn and drives everything after. Beckett spends the rest of the film grieving in a register the audience is not invited to examine closely.
The last lesson
He betrays Han at Savareen, exactly as advertised, and Han shoots him first — the moment the film exists to deliver, and a deliberate inversion of the Mos Eisley cantina argument that has occupied the fandom since 1997. Beckett dies having been proved right about everything except who would do it to whom.