Nimue, more frequently referred to by her title The Lady of the Lake, is the alternate reality doppelgänger of Amy Rose from the world of Camelot, who appears in Sonic and the Black Knight. She is a skilled enchantress, and former guardian of the sacred sword Excalibur.
At Merlina's behest, Sonic and Caliburn journey to Nimue's home in Deep Woods in their quest to determine how to blunt the power of Excalibur's scabbard. Sonic is predictably unnerved by the Lady's resemblance of Amy Rose, but Caliburn remonstrates the hedgehog and they enter the laketop pavilion. During a conversation with her, Sonic seems to flirt with Nimue, and she seems noticeably startled.
Nimue blames herself in part for the present state of the Grand Kingdom, as she was the one who initially gave Excalibur to King Arthur, only for the power of the scabbard to corrupt the once-noble King. She agrees to help Sonic only if he can prove himself a true and honorable knight who will not be similarly bewitched, and assigns the hedgehog three tasks to demonstrate his chivalry.
After Sonic has completed the first task, liberating King Arthur's prisoners in Camelot Castle, Nimue magically disguises herself as a distraught child whose fellow villagers have been kidnapped by a dragon. The hedgehog's heroism in saving the Arthurian folk convinces the Lady of Sonic's just intentions, and she tells Sonic how to negate the scabbard's power using the sacred swords wielded by the Knights of the Round Table.
Later, during Sonic's battle against the Dark Queen in the Dark Hollow, Nimue's reflecting pool allows her and the Knights to communicate with Sonic and Caliburn.