Méndez is the name of a successive dynasty of mutant, human leaders, in the Planet of the Apes movie universe. Actor Paul Richards played Méndez in 1970's Beneath the Planet of the Apes, while Paul Stevens played his predecessor in 1973's Battle for the Planet of the Apes, the final original Apes movie.
In the timeline of the first two Apes movies, Méndez is a figure much like a Pope or other lineal authority, with his leadership basically spiritual in nature. His people are the descendants of survivors of a nuclear war, which destroyed most of humanity and allowed the apes to rise to power. Living underground for centuries among irradiated ruins has transformed them physically; their psychic powers increased, as their appearance became disfigured through severe genetic mutation. The underground mutant people wear masks and wigs to resemble their ancestors more closely, and speak through telepathy, saving their voices for worship. They also regard their severe physical mutation as a true blessing of "the divine bomb". During their worship ceremonies, the mutants cast off their contrived masks, revealing their "inmost selves" unto their god, and much of their speech and daily rituals are stylized around terms used within the nuclear industry.
Their object of worship is an ancient "Holy Weapon of Peace"; an Alpha-Omega nuclear missile left over from the 20th century and still operational, though its original controls have long been replaced by carefully-crafted jewel and crystal workings. They see their life's purpose as to guard the Divine Bomb, and to keep watch on the apes; should the apes become a threat to their underground life, the Bomb will be used to destroy them. However, what is little understood is that the Alpha-Omega Device - which possesses a cobalt casing around its warhead - was designed to ignite the Earth's atmosphere, and extinguish all life on the planet, not just the apes.
When Taylor and Brent are captured, Méndez oversees their interrogations, and decides what is to be done with them, and about the apes, who are planning to invade the underground city to seize its food sources. When the apes arrive, Méndez tries to reason with their leader General Ursus, but is shot down after arming the missile. Taylor later detonates the missile's warhead, ending the battle between human and ape once and for all by destroying the entire planet.
Set almost 2,000 years before Beneath, Méndez in Battle is the first of the underground humans to bear the name, and is a subordinate to Kolp, who became Governor after Governor Breck's death, following the nuclear war. When Kolp goes to battle with Ape City (made up of apes and humans who fled the Forbidden City before the war) with his mutant army, Méndez remains behind, supervising the team who safeguard their "secret weapon"; the Alpha-Omega missile.
When Kolp loses the battle, the default order is to fire the missile at Ape City. Méndez instead rejects the order, reminding everyone that using the missile will not just destroy the apes. If they instead revere its power and preserve the missile through time, they will never lose hope or a sense of purpose. He becomes the new human leader, and his attitude toward the missile becomes the code of the underground humans, who build their society to reflect that code. His successors in turn carry his name, as a reminder of their purpose.