The son of the Hulk, Skaar, once saved the Avengers from a dark takeover that was spearheaded by Norman Osborn. In the New Avengers/Dark Avengers storylines, Skaar was recruited by the villain to be part of a new team of Avengers that was created to overtake the main team and turn the public in their favor and against the longtime heroes. Osborn's plan almost worked, but it turned out Skaar was actually working as a double agent from the getgo.
Skaar is the powerful son of the Hulk, who was secretly born after the Jade Giant left Sakaar following the death of his love Caiera. Skaar not only has incredible strength but was born with his people's Old Power, which gave him a wealth of cosmic-esque powers that dwarfed his father's abilities. Skaar would eventually make his way to Earth and battle his father, before becoming an ally to him. Skaar was ultimately recruited to the second version of the Dark Avengers, where he proved to be quite helpful to the real Avengers.
In New Avengers #23 by Brian Michael Bendis, Mike Deodato, Will Conrad, Rain Berdo, and VC's Joe Caramagna, Norman Osborn gained quite a bit of power and influence in the Marvel Universe, controlling H.A.M.M.E.R., which combined some of most evil organizations together, including Hydra and the A.I.M. In order to appeal to the public, he creates a new Dark Avengers team to rival the Avengers, which includes Toxic Doxie as Scarlet Witch, Gorgon as Wolverine, and Ai Apaec as Spider-Man. Osborn and the Dark Avengers turn the public against the Avengers - but as he's about to take down the team, Skaar reveals he's been working alongside Captain America from the very beginning.
Skaar proved to be the secret weapon the Avengers needed to stop their dark counterparts, as he took down the Dark Avengers singlehandedly, and freed Captain America, who was captured by Osborn. Without Skaar's help and immediate call to Captain America after being recruited, there's no telling if Osborn's Dark Avengers team would have supplanted the original group as Earth's Mightiest Heroes due to their favorability to the public. Skaar put out the fire before it burned the Avengers down for good.
Skaar might be mostly forgotten in the comics today, but during the arc in New Avengers, he saved the day by teaming up with Captain America as a double agent on the Dark Avengers. Without the son of the Hulk by their side, the Avengers might have lost their battle against Osborn and been overtaken by their evil counterparts. Thankfully, Skaar was on their side and helped save the day.