The character who kills Severus Snape is Nagini.
Near the end of Harry Potter and the Deathly Hallows, Voldemort summons Snape to the Shrieking Shack. Voldemort believes that Snape is the master of the Elder Wand and mistakenly thinks that killing him will make the wand obey him. Instead of using the Killing Curse directly, Voldemort orders Nagini to attack. The snake bites Snape through the neck, fatally wounding him.
Snape survives long enough to release his memories to Harry. Those memories reveal that he had loved Harry’s mother, Lily Potter, and had secretly worked against Voldemort for years. They also explain why Dumbledore trusted Snape and why Snape accepted blame for Dumbledore’s death.
Nagini’s role is especially important because Voldemort had made her one of his Horcruxes. After Snape dies, Harry learns that Nagini must also be destroyed. Neville Longbottom later kills the snake with the Sword of Gryffindor during the Battle of Hogwarts, removing the final protection that helped keep Voldemort immortal.