Kavil is one of the tiefling refugees seeking shelter at the Emerald Grove in Act One. She only appears as a replacement character for Bex during the camp celebration if he is dead.
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Grab a drink! I'm fairly certain it's all yours anyway.
Kavil will only appear at the tiefling camp celebration, which happens during a long rest after successfully defeating the goblins threatening the Emerald Grove. She takes the place of Bex if she is dead.