Kali is an extremely important, yet feared deity within Hindu belief systems and mythology, as well as countless cultural folk tales and stories based on the rich tapestry of these texts, with India having some of the oldest and most memorable tales of good versus evil and heroism (such as the famous Ravana and the invincible Mahishasura).
She is not to be confused with the demon known as Kali, which is an apocalyptic figure destined to face the tenth (and final) avatar of Vishnu according to some Hindu traditions.
Kali is a goddess, and as such, is an aspect of a greater divine purpose. (Despite the apparent polytheistic nature of Hinduism, most of their major deities are said to be simply avatars of a single monothestic force, which humans are incapable of grasping in full.) However, Kali embodies destruction and divine wrath.
Kali's role is thus to seek out evil and destroy it, a role she shares with Shiva (her male counterpart, who is often depicted as her consort). While this means she is a force of good, she is nevertheless a deity demanding respect and fear as she can perform any task in order to achieve this goal, and is known for her bloodlust.