Lucia, along with Dante, is one of the two primary protagonists in Devil May Cry 2. An agile fighter, she uses two ornately-carved curved daggers. Like Dante, she can Devil Trigger (transform into a "harpy," a birdlike demon of light).
Lucia is a member of Vie de Marli, a clan of guardians with the blood of devils. She invites Dante to her island so her adoptive mother, Matier, can ask him to help them defeat Arius (a man who has turned their land into a devils' paradise). Dante accepts, and he and Lucia begin their quests. The player later learns that Lucia is actually Chi, a devil bred by Arius but abandoned as defective. When she reaches Arius, Dante has already defeated him and rides his motorcycle to the demon world to stop an ancient demon. After Dante leaves, Arius (now a monster) rises from the ruins and attacks Lucia; she defeats him, and awaits Dante's return.
Source: Wikipedia, "[en.wikipedia.org/wiki/Characters_of_Devil_May_Cry%23Lucia Characters of Devil May Cry]", available under the CC-BY-SA License.