St. Medard, Bishop of Noyon, France, is counted as an intercessor for persons with mental illness. A very godly child, he was, reluctantly, ordained a bishop at age 33. He was, apparently, a very zealous and popular bishop.
But there are few details known of his life. He is known for having been protected under the wings of an eagle during a rainstorm as a child. He is commonly depicted as laughing, with mouth wide open.
If I may be so bold to speculate as to why he is counted as an intercessor for people with mental illness, I wonder if his being sheltered from the elements as a child relates him to the fools for Christ who, feigning madness, live in the open, and, like Christ, have no place to lay their head, as well as to all who, for true lack of mental health, are homeless.
Source of picture: http://jfelker.blogspot.com/2011/05/prayer-to-st-medard-patron-saint.html