Wizard and Glass
Jonas is rather reticent about his past, though it is plain that he is a man of considerable experience. Coral Thorin notices his back is covered in scars, and expresses amazement that he had even managed to survive such a flogging, but nothing is known of this particular incident other than Jonas' own claim that it happened in Garlan, a country distant to Mejis. It seems he may have had palaver with the Manni for he knows of traveling between worlds.
He along with Clay Reynolds and Roy Depape allied themselves to John Farson when they became the Big Coffin Hunters. By the time of his appearance in Wizard and Glass he is old enough to have a wall of white hair down his back, but remains a formidable opponent, either on his own or at the head of his own ka-tet of lesserly qualified thugs, the Big Coffin Hunters consisting of him, Roy Depape, and Clay Reynolds.
Jonas' acts suggest that there was more to his failure to become a gunslinger than a lack of physical proficiency. Although cunning and resourceful, he never once expresses any higher feeling. His objection to Depape tormenting Sheemie was not out of concern for Sheemie but anger that Roy had gone about it in public. By his own admission his sexual relationship with Coral Thorin is no more 'love' than that of a pair of rutting donkeys, and it would appear that any connection he has with her is down more to them sharing a similar amoral world-wiew. (This stands in severe contrast to Steven Deschain's admonition to Roland regarding the importance of feeling in The Gunslinger.) One of Jonas' attacks upon Roland's own ka-tet involved tearing up a book and scrawling rude words while Roland's ka-tet were otherwise engaged. The unrestrained glee Jonas expresses while doing so is highly unbecoming for a man approaching pensionable age, and even more so for someone who was once considered gunslinger material.
In Hambry, Jonas is employed as an enforcer by the Barony authorities, all of whom are aware to a greater or lesser extent of his true allegiance and purpose as an agent of John Farson. He dies at the hands of Roland after Maerlyn's Grapefruit is taken from him.