Kali is the goddess of destruction. She claims to love Yama, and she and Sam have a history together, but in reality, she really "cares only for those who bring her gifts of chaos" (3.474). Boy, Pat Benatar was right on the money with Kali when she wrote "Heartbreaker."
When Kali's offered the position of Brahma, and a chance to create a new war with Accelerationism, she jumps at the opportunity, ending her marriage with the lovesick Yama. She remains Brahma until her near death injuries at the Battle of Khaipur, when Yama transfers her atman into a child's body, accidently damaging the soul in the transferring process.
Kali is an antagonist in the novel because she basically stands in complete opposition to everything Sam stands for.
Whereas Sam is steadfast and constant in his aims, Kali is inconsistent, leaping from one primal emotion to the other, one desire to the next. For example, in the span of a single conversation, she claims to want to forgive Sam and "invok[es] the seven Rishi to bring [his] image before [her]" to "desir[ing] his death" (5.94). In the same vein, she pleas with Brahma to spare Sam's life only to later ask Brahma to serve Sam's life up as a sacrifice for her wedding. And here we thought the tradition was something old, something new…
Although she fights against Accelerationism and takes up Brahma's form, she doesn't truly believe in the anti-accelerationist cause—she would just as fiercely join Sam's cause. Why? Because as she herself says, "Kali casts her allegiances where she will, owing nothing to anyone, but as she chooses […] a mercenary goddess" (5.109). She follows her battle lust wherever it may lead her; violence for violence's sake is her true love, not any particular cause.
Once again this is the complete opposite to Sam's character, since he only takes up causes—violent or otherwise—because he truly believes they provide the right course for humanity.
Kali's name means "the Black One," and she is the Hindu goddess of destruction and dissolution and so is a companion to Shiva the destroyer. Kali is a figure that can be seen as both terrifying yet necessary—on the one hand, there's that whole destroying part of her career, but on the other hand, she maintains world order and frees humanity with the blessing of death. Interestingly, she's also the mother goddess (source).