Diego is a saber-toothed tiger and one of the leaders of the Sub-zero Adventure Squad.
He has a somewhat sarcastic personality, although it's usually not intended as malicious. He was sent by his pack to retrieve the human baby; When he found the baby in Manny and Sid's care, he pretended to lead them to the humans, but instead planned to lead them into an ambush by the pack. However, when Manny saved his life Diego changed his mind, telling him about the pack and helping to counterattack. He appeared to have been killed by the pack's leader, Soto, but survived and joined Manny and Sid on their travels.
In the second film, he was revealed to suffer from hydrophobia, but he eventually conquered it in order to save Sid, Eddie and Crash.
In the third film, he began to fear that he was losing his predatory abilities from being in the "herd" too long, and considered staying in the underground dinosaur world. He changed his mind after managing to save the labored Ellie from a dinosaur attack.
In the fourth film, his first love interest is Shira, a saber-toothed tigress who had originally worked with Captain Gutt and his crew.
In The New Adventures of Ice Age: Dawn of the Dinosaurs and the Manny, Sid, and Diego's Adventures Series, his second love interest is Tigress, despite the fact that they're both (and called) tigers.
In Manny, Sid, and Diego’s Adventures of Monsters vs. Aliens, he saved Tigress from Rudy’s snapping jaws and she done the same thing for him when he falls into a lava pit in Manny, Sid, and Diego’s Adventures of The Land Before Time.
He will make his guest appearances (along with Manny, Sid, and the Sub-zero gang) in other projects outside of the Manny, Sid, and Diego's Adventures Series, such as Danny Phantom's Adventures of Short Circuit 2, Danny Phantom and Shrek's Adventures of Rio, and several episodes in Captain Charles T. Baker's Adventures of Kim Possible.
He is voiced by Denis Leary.