Kasdan had little to do with the mentor idea. Like just about everything Star Wars in cinema from 1977 to 2005, it came from George Lucas. Han's stepfather appeared in the first, rough draft for Empire Strikes Back, written by Leigh Brackett. Kasdan would be brought in after Brackett's untimely death. The stepfather's name was Ovan Marekal and he was an extremely powerful figure in the galaxy. According to the draft, only the Emperor and Vader were more important. George had come up with a back story for Han and his stepfather during story conferences with Leigh in late 1977. From those notes:
(From The Making of The Empire Strikes Back)
In the early drafts of Empire, Leia approaches Han early in the movie to talk to his stepfather about allying with the Rebellion. Han initially refuses, but by the end of the script he agrees and it ends with him and Chewie flying off in the Falcon to find the stepfather.
George Lucas said:Han's stepfather is a Hemingway-esque character. His father was a trader and his father's father was a trader, and they honed out this trading post-empire and pretty soon it became this giant thing. In his trading, he came across Han and took him under his wing for eight or nine years- until they had a falling out at the end and had a bitter fight over something. Han swore he would never talk to him again.
(From The Making of The Empire Strikes Back)
In the early drafts of Empire, Leia approaches Han early in the movie to talk to his stepfather about allying with the Rebellion. Han initially refuses, but by the end of the script he agrees and it ends with him and Chewie flying off in the Falcon to find the stepfather.