Shinobi

Shinobi was a very powerful Elf. He was Rex's great-great-grandfather.

Pre-Series
A long time before Rex was born, Shinobi was the leader of the Elves. But when the ceiling of the catacombs under Pixieville collapse, Shinobi became crushed beneath the rubble.

Series
When Rex fell into the catacombs, he saw the the staff and the only thing remaining of his great-great-grandfather: the bones of his right arm. After he climbed out of the catacombs, he showed the power of the staff to the other Elves and destroyed the bones of the arm and made a plan to conquer Pixieville with the "legacy" of Shinobi.

Magical Abilities
Shinobi used a very powerful staff which increased his powers. His staff is the completely opposite of Ninfea's staff: Her power is mostly generated from light, while his power is mostly generated from darkness. Ninfea and Shinobi might be very matched to each other.

Trivia

 * Shinobi is another name for Ninjas. In ancient Japan the shinobi was a covert agent or mercenary in feudal Japan who specialized in unorthodox warfare. The functions of the ninja included espionage, sabotage, infiltration, and assassination, and open combat in certain situations. Their covert methods of waging war contrasted the ninja with the samurai, who observed strict rules about honor and combat.
 * Shinobi being the great-great-grandfather of Rex recalls the Ancestral Witches, who are the ancestors of the Trix, who are the recurring villains in Winx Club.