The Magna Defender is a fictional character from the Power Rangers: Lost Galaxy TV series. He is a warrior who has been alive for many years, and is the sworn enemy of Scorpius.
Later in the series, the Magna Defender passed his powers on to Mike Corbett. Throughout the rest of the series, Mike was known as the Magna Defender when he used the powers.
Years later, Red Galaxy Ranger Leo's brother Mike fell into the same pit. The Magna Defender fused with him, escaping the pit and setting out after Scorpius and the legendary Lights of Orion. Using his Torozord, he fought several battles against his enemy. Obsessed with revenge, he had no qualms about going through the Rangers or even civilians in the pursuit of his vengeance, and so was a frequent foe of the Rangers despite their common enemies. Later, while attempting to destroy Scorpius, (in a last-ditch attempt that would have most likely destroyed Terra Venture as well) the Magna Defender was wounded and revealed to Leo that he was merged with Mike. Seeking to undo the damage he caused to Terra Venture in his attempt to finish off Scorpius, and after being prompted to return to good and renounce his hateful ways by an apparition of Zika, Magna threw himself into the energy pit he had created, sacrificing his life to neutralize its energy and save Terra Venture. In the process, Mike was released.
The spirits of Magna and Zika later appeared to Mike when the Rangers were in peril, giving him the Magna Blaster in Sword mode, the source of the Magna Defender powers. Much like with the Quasar Sabers, a two-part morpher also appeared on Mike's arms, allowing him to become the new Magna Defender. This two-part morpher was actually the morpher from Gosei Sentai Dairanger, the sentai series from which footage was lifted to create the second season of Power Rangers. Using the Magna Blaster and Torozord, Mike did battle with many foes of the Rangers.
Later, Mike sacrificed Torozord and his powers to free Terra Venture from the Lost Galaxy. Then the original Magna Defender's spirit appeared to Mike again, this time to congratulate him on how heroically he had used the powers and tell him that he had fulfilled the Magna Defender's destiny. Prior to Mike's sacrifice of the powers, Magna Defender's spirit appeared to him and motivated him to perform the action.
Note that while Mike was able to morph into the Defender, the original Defender was never seen outside his armour, so it's impossible to say if the armour was actually his body. Zika was also only seen in armour (a child-size replica of his father's armour).
There has been some criticism of the characterization of the Magna Defender prior to his death. While it is not conclusive, the flash backs in the series seem to indicate that the Lights of Orion were originally owned or at least guarded by the Magna Defender. It is also clear that anyone could possess the Lights, but the Lights are still used as a plot device to pass judgment on Magna Defender when they "choose" the Power Rangers over him, purportedly because of his anger and resentment toward Scorpius. Also, during the battle with Fishface, who has a hostage, the Magna Defender continues the fight despite the presence of non-combatants, only to have the Red Ranger attempt to restrain him. Fishface attacks and injures both Magna Defender and Red Ranger until the hostage escapes. The show portrays Magna Defender's acts as wrong and as a result of his uncontrolled anger, but no serious attempt is made by the show to analyze Magna Defender's actions. At the time, Fishface was believed to be close to discovering the immense power of the Lights of Orion, which would have given him enough power to become a real threat. Throwing down one's weapons in the face of a hostage situation makes little sense when it would allow the enemy to do even greater harm, but the show makes no attempt to delve into this quandry. Indeed, eventually the Power Ranger's meddling results indirectly in Magna Defender's death, but afterward his spirit returns and ratifies the Power Ranger's conduct, thus leaving viewers with the sense that the Power Rangers were right all along without ever having been forced to defend the morality of their actions. This criticism can be generalized to many Power Ranger incarnations, but is starkly illustrated in the Manga Defender saga.