He is an altered gorilla who was originally a member of the Ape Army, being one of Koba's followers, who believed what he did was good. Following Koba's death, he defected as he found it weird how Caesar got the leadership after Koba died, and left the humans alone.
After a while Red got tired of this, and talked with Grey, and wanted to leave Caesar's army, Grey agreed and they started creating their own colony and the other apes that used to support Koba before his death, ultimately, Grey became the leader, but Red was the brain behind all of their decisions.
Red wanted war against the humans, he wanted it so much that the leader, Grey, turned on Red. This was because they were going to kill Cornelia, during the birth of one of the females.
Eventually, Red joined the Alpha-Omega military faction of the human army and became Colonel McCullough's "pet", personal bodyguard and third-in-command. Red has a tattoo on his back that says: "DONKEY", a nickname for the Apes who serve the human army.
War of the Planet of the Apes
At the beginning of the film, Red accompanied a group of humans as they launched an attack on the ape army. However, he was captured and brought before Caesar who declared that he fought to defend his people. When a human soldier pointed Red out, Caesar looked at him and stated he was one of those who served humans because he fear him but Red denied this as another ape struck him with a gun.
Red was then interrogated by Winter who he overpowered and demanded the location of the Ape Colony hideout. Realizing that in order to live, he had to give up information, Winter agreed on the condition that he be allowed to serve the Colonel, because otherwise he would slaughter and torture him like he would do with the other apes who weren't "DONKEYS" (Apes that serve humans). Red agreed to the terms and Winter gave it up Colonel, because otherwise he would slaughter and torture him like he would do with the other apes who weren't "DONKEYS" (Apes that serve humans).
When Caesar noticed this, he got angry, the Apes saw him as a feral untrustworthy traitor. Even though being part of the Apes once, Red was extremely brutal and aggressive when torturing the Apes, most likely more than the humans. A good example of this was when every Ape got water, and when he was gonna give Caesar water, he poured ice-cold water on him instead.
Even though all of this, he still fought for the Apes in the end of the movie. When giving the troops their ammunition, he saw how much Caesar struggled, all the pain he had felt, and he saved Caesar and the rest of the Apes by killing Preacher and allowing Caesar to destroy the gas canister, but Red was shortly killed afterwards and didn't resist when it happened.
By the end, the human military was left weakened and the apes lived to see another day and rebuild.
He is the only gorilla to speak in Planet of the Apes reboot franchise.
The name Donkey which the Alpha-Omega soldiers calls him is a reference from Donkey Kong.
His appearance is similar to that of Blizzard from the Game Primal Rage.