Imperative
Necessary, important, unavoidable
Impertinent
Rude
Implacable
Can not be soothed or won over
Incorrigible
Bad beyond correction
Incredulous
Skeptical
Indigenous
Characteristic of a particular region
Indigent
Lacking basic necessities of life because of poverty
Inhibit
To restrain or prohibit
Innate
An integral part of something; natural behaviors or characteristics
Innocuous
Harmless