Negligible. That's the word I want!

 The butler smiled indulgently. He did not understand what Reggie was talking about, but that did not worry him. He had long since come to the conclusion that Reggie was slightly mad, a theory supported by the latter's valet, who was of the same opinion. Keggs did not dislike Reggie, but intellectually he considered him negligible.

(from A Damsel in Distress, by Sir Pelham Wodehouse)

Wodehouse occasionally will use "negligible" to describe characters who are not all they should be mentally, and it works very well. I wish I had thought of it. (I hope he didn't copyright it.)