An implacable foe
An inborn and irresistible impulse
An incongruous spectacle
An incredible mental agility
An indefinable taint of priggishness [priggishness = exaggerated propriety]
An indescribable frankness and simplicity of character
An indolent surrender to mere sensuous experience
An indomitable and unselfish soul
An ineradicable love of fun and mystification
An inevitable factor of human conduct