Ack is seen partaking in the First Dragon War, defending Berk against the marauding dragons. He sees a young Hiccup trip outside during an attack when he should be inside. Ack wishes him a good morning.
Anyone else want to try being in charge? Starkard? Ack? Mrs. Ack?[src]
According to the graphic novel, The Serpent's Heir, in the aftermath of the attack by Drago Bludvist, the Scuttleclaw Hatchlings seen in the second movie has taken up residence on Berk. Hiccup, now Chief, leaves Gobber in charge while he leaves to help the people of Nepenthe. The brood of Scuttleclaws wreaks havoc on Gobber's smithy, chewing on everything and not listening. Gobber laments for someone else to take charge, and names Ack, Mrs. Ack, and Starkard.
Ack is a large built Viking that is easy to spot in battle when he takes up arms against the dragons.
Ack appears to be a slightly older adult Viking with a white streaked blonde beard. He is missing some teeth on his lower jaw and like most Viking men on Berk, he wears the usual Viking attire and a proud Viking Helmet.
Like most men of Berk’s Viking village, he is a fighter whose energy and enthusiasm translates to unparalleled bravery on the battlefield. Despite his intensity, Ack is mindful of certain “battlefield etiquette” as he is known to stop in the midst of a conflict to courteously greet others before resuming the fight. Much like his fellow Vikings, Ack’s wild mannerisms convey his deep devotion to his kinsmen as he defends for his home and beloved leader, Stoick.
Ack seems to be able to switch easily between wild, aggressive battle mode to a genial non-aggressive greeting in seconds, then back again, as seen in the first movie when he knocks down Hiccup and nearly splits him in two, then smiles and says ARGGGHHHHH! Mornin'!.