Elton Thumps is Leven's father. In book five, it is mentioned that Leven sees a dream of Elton's, and so learns that his father is still alive. Despite desperately wanting to meet him, Leven does not until he reaches the island of Alder (Elton is part of his "unfinished business"), where they try to kill each other.
Unbeknownst to Leven however, Elton is introduced in book four, where he takes an interest in Dennis, Ezra, and the happenings at Blue Hole Lake. After a series of actions and conversations, it soon becomes clear that Ezra does not wish to work alongside (or, indeed, have anything to do with,) Elton, so of course, Elton imprisons Ezra in a glass jar.