informal, natural, unconventional, and careless
insolent, impudent, impertinent, and flippant
integrity, frankness, sincerity, and truthfulness
intellectual, moral, emotional, and esthetic
intense, earnest, violent, and extreme
invent, discover, design, and contrive
inveterate, confirmed, chronic, and obstinate
invidious, envious, odious, and offensive
invincible, unconquerable, insurmountable, and insuperable
irksome, tiresome, tedious, and annoying