Great Leader of Shocker

The Great Leader of Shocker is the primary antagonist of the orignal 1971 Kamen Rider series. The leaders of other evil organizations took influence of him, such as Destron's Great Leader.

Physical Appearance
The Great Leader wears a red robe and a medallion with the Shocker logo in the center. He has a Medusa-like snake head that is actually a mask covering his true cyclops head.

History
His ultimate goal is to turn the entire human race into kaijin under his rule in order to unite the world as one being. He orders his subordinates via speakers on Shocker's emblem to destroy Kamen Riders Hongo and Ichimonji who are the obstacles of his conquest. The two riders were able to severely weaken the Shocker organization by eliminating most of their key figures (Colonel Zol, Doctor Shinigami and Ambassador Hell), this forced the Great Leader to reform Shocker by allying himself with General Black of the Geldam organization, thus creating Gel-Shocker.

Seeing that old members of Shocker were considered unfit for the new organization, the Great Leader had them all killed one by one. All Shocker secret bases were self destructed and the remaining Shocker soldiers were killed. The Riders were able to defeat Gel-Shocker forces and its kaijin's, including General Black. The riders were able to locate Gel-Shocker's headquarters and finally confronted the Great Leader himself, the two riders uncloaked the Great Leader only to reveal that he is actually a Cyclopean medusa. He sets off the headquarters' self-destruct system in a final attempt to kill the riders, but only resulted in him being killed instead and the two riders escaping, therefore ending Shocker's reign of terror. Or so they thought...

Remergences
Shortly after, the Great Leader resurfaced as the Great Leader of the Destron organization. In response, the Double Riders creating the third Kamen Rider, V3. He would go on to create several more evil organizations to menace the ever-increasing amount of Kamen Riders. His last known appearance was as the Generalissimo of the Badan Empire, who disappeared shortly after he made himself known to the ten veteran Kamen Riders. He may have also been the Grand Lord Crisis of the Crisis Empire who was destroyed by Kamen Rider Black RX.

OOO, Den-O, All Riders: Let's Go Kamen Riders
The Great Leader makes a return in the movie OOO, Den-O, All Riders: Let's Go Kamen Riders, serving as the film's main antagonist. During a small conflict between the characters of Kamen Riders OOO and Den-O during their time travel in Japan during the 1970s while hunting the Imagins, General Black discovered a Cell Medal that was accidentally dropped by Ankh during the conflict. General Black gave the medal to the Great Leader which he combines with the Shocker medal in order to create Shocker's most powerful kaijin, Shocker Greeed. The kaijin was able to defeat both Hongo and Ichimonji, thus causing a time paradox that created a world where Shocker reigned supreme. Various evil organizations that would have manifested in the normal timeline after Shocker's destruction later affiliated with Shocker and the Great Leader such as, the Delza Army, the Crisis Empire and the Government of Darkness. Shocker also gained control of both Hongo and Ichimonji via brainwashing.

OOO and Den-O returned to 1970s Japan in hopes of stopping Shocker Greeed from being created and returning the timeline back to normal. The Great Leader was confronted by the Riders, but was able to create Shocker Greeed and the kaijin manage to take down the Riders. Having failed to restore the timeline and the DenLiner destroyed, the Riders retreated back to the Shocker-ruled future and decided to confront Shocker, but were overwhelmed by the various kaijin forces. As the Riders were about to be publically executed, Hongo and Ichimonji (having been faking their brainwashing in order to wait for the right time to fight Shocker) summoned all the other Riders and were able to defeat the kaijin army.

The Double Riders have a rematch with Shocker Greeed while the other Riders rushed toward Great Leader. Shocker Greeed is destroyed by the Rider Double Kick before Ankh takes the Shocker Medal. When the Riders are overpowered by Great Leader's attacks, Ankh convinces OOO to use the Shocker Medal and an Imagin Medal from Momotaros in combination with the Taka Medal. With the three Medals, OOO changes into Tamashii Combo and destroys Great Leader with the Tamashii Bomber. The attack, however, unleashes Great Leader Colossus - the true form of Great Leader, who throws fireballs at the Riders and consumes the other Shocker leaders in an earthquake. The Riders are joined by Kamen Rider Birth and all of the extra Riders to battle the new menace. All of the Riders jump into their motorcycles to execute the All Rider Break, destroying Great Leader Colossus once and for all.

Kamen Rider Decade
For the more information on the Great Leader's Decade Incarnation, see here

In the Heisei period film Kamen Rider Decade: All Riders vs. Dai-Shocker, Tsukasa Kadoya (Kamen Rider Decade) was the Great Leader of Dai-Shocker, before he was overthrown by Shadow Moon. Tsukasa later became Great Leader again during the events of Kamen Rider × Super Sentai: Super Hero Taisen, before revealing that he was actually working to sabotage Dai-Shocker's Big Machine operation.

Powers and Abilities
Great Leader's primary ability is summoning gas from his body to knock out his foes. and whilst he never did this in the original series, in the Let's Go, Kamen Rider movie, he could use the snake heads as tendrils coming from his head and shoot fireballs from ths snake's mouths.

Also in that film, he became his greatest form, the Great Rock Leader, one of the largest entities in both Kamen Rider and Super Sentai (one of the closest is Black Cross Fuhrer).

The Great Leader is also seemingly unkillable, as any time he is defeated or destroyed, he returns in a new form or resurfaces to antagonize the Riders regardless if he was defeated before. (Usually by time travel or distortions in space/time)

Trivia

 * The Great Leader bears some similarities to Demise as both have more than one incarnation and are known to be "hijacking the story" though the former has more incarnations than the later.
 * The Great Leader's seemingly immortal presence has vaguely been explained and has multiple variations:
 * Kamen Rider co-creator Toru Hirayama stated in an interview in the Uchûsen Book Collection Special Edition that the Great Leader was once a monk named Chan Mao who discovered the secret of transcending death. This led him to be detached from humanity and eventually drove him to lose his own, as he became an evil demigod-like figure who wanted to rule the world.
 * The series Kamen Rider Stronger contradicted this by showing that the Great Leader was in fact an alien cycloptic brain. This was again contradicted when it was revealed in Skyrider that the Great Leader was a giant dragon.
 * In the Manga Kamen Rider Spirits, he is called JUDO and is an Eldricth Monster who seeks to inhabit a vessel in the form of a cyborg human body.
 * The last explanation is a specualtive one by fans based on a line from Kamen Rider Black RX, "As long as Humanity's evil exists, there will be a new Crisis Empire! And I will return to lead it!". This could mean that the Great Leader somehow uses the human race's own evil and hatred to sustain and regenerate himself.