Direct text from devil may cry wiki:
Rebellion once belonged to Dante's father, the Dark Knight Sparda, and was given to Dante as a keepsake. Dante uses it as his primary weapon in all appearances, barring the first Devil May Cry and his cameo appearance in Viewtiful Joe. Dante usually carries the sword on his back, but in The Animated Series, he carries it in a guitar case, so as not to attract unwanted attention.
At the end of Dante's first battle with Vergil in Devil May Cry 3, Vergil impales Dante with the Rebellion, inadvertently awakening Dante's Devil side and granting him access to Devil Trigger. At this point, the sword also awakens, opening up the crossguard and skull design around the hilt.
