Sentinel Knight

The Sentinel Knight is the guardian of the Corona Aurora, a powerful crown

Biography
After Flurious and Moltor tried to steal the Corona Aurora and were cursed, the Sentinel Knight hid the crown and its five jewels throughout the planet Earth. His spirit returned to urge that Andrew and his team of Rangers protect the crown from impending forces. He once battled and imprisoned Thrax, the son of Rita Repulsa and Lord Zedd.

When Thrax disabled the Overdrive Ranger's powers, the Sentinel Knight summoned five veteran Rangers and repowered them so they could temporarily take the current Rangers' place. After Mack retrieved the sword Excelsior, he gave it to the Sentinel Knight so that no one could use it to harm him and its power restored Sentinel Knight to a physical form.

After being restored to a new physical form, he swiftly destroyed Thrax. He has since been capable of transforming into the powerful Sentinel Sword weapon. Thanks to modifications by Rose, Sentinel Knight was able to merge with Mack to become the Red Sentinel Ranger. When the Corona Aurora was successfully retrieved, the Sentinel Knight took his original form. Thanks to him, Mack became human by placing the crown on his head.

Sentinel Knight Transformation
This is the Sentinel Knight transforming from his sword mode also known as The Sentinel Sword.

Trivia

 * While he does have a Sentai counterpart, his original "spirit" form was not in Boukenger.
 * The Sentinel Knight plays a similar role to Ninjor from MMPR's Season 3.