In A Wrinkle In Time, Mr. Jenkins is the principal of the high school that Meg Murry attends, a minor character.  He is a major character in A Wind in the Door, at which point he has been transferred to become the principal of the grade school that Charles Wallace Murry attends.

Jenkins is poorly regarded by most of the students, and has an especially antagonistic relationship with Meg, who is sent to see him far too often so far as either of them is concerned,  Jenkins does not understand children very well, and uses a vocabulary inappropriately advanced for them when he deals with them.  In A Wind in the Door, we come to understand him better as a good man whose kind heart is buried under years of frustration and loneliness.