Maximillian Acorn (or just the King, as he was referred to in the Saturday morning cartoon (in which he was voiced by Tim Curry) is the father of Sally Acorn. He is an anthropomorphic, humanoid squirrel, and his current age appears to be somewhere between the mid 50s and early 60s. He appears to be more of a squirrel than Sally, as he has a longer, more bushy tail. He is also discreetly taller than her. The Acorn Family Ring is the only known belonging of the king (presumably given to all members of the royal family). He knows the location of all the Freedom Fighters and knows secrets about the planet that others haven't heard of, such as the Deep Power Stones.
Maximillian Acorn was the ruler of Mobotropolis in the sovereign Kingdom of Acorn, until the evil genius Dr. Robotnik took over and banished King Acorn to the barren interdimensional realm known as The Void. Sonic and Sally later used the Time Stones to travel back in time, meeting him and warning him of the threat, but they were unable to save him from this imprisonment, for it was the very same day that the coup took place. Worse yet, the King was not alone; the Void was also the prison of Robotnik's mentor Naugus, a dark wizard of great power.
Sonic and his allies managed to enter the Void years later, finding the King alive and well. However, he only escaped the Void for a brief time with them before his body began to crystallize as a result of years of exposure to the Void's strange environment. As a result, he was forced to return there, much to the dismay of the Freedom Fighters and Sally in particular. However, they did benefit from the brief meeting: not only did they rescue the King's fellow prisoner Ari, but they also received a list of all their ally groups.
Had the series not been canceled, Maximillian Acorn would have escaped the Void along with Naugus, and been used by the wizard as bait to lure the Freedom Fighters into a trap.
Maximillian Acorn's personal timeline in the series is distinctly skewed. Shown to have been imprisoned in the Void on the same day as Robotnik took over Mobotropolis, he is later shown to somehow possess a list of all known Freedom Fighter groups - how could he have such a list when the Freedom Fighters did not exist when he was banished? Additionally, the first episode of the series indicates that he was until recently held in Ironlock Prison, from which he transmitted sensitive documents to his daughter - an event that is totally incompatible with what would later be revealed as his fate.
The series echoed SatAM in the fact that both King Acorn and Ixis Naugus were banished, but to the Zone of Silence rather than the Void. While in the Zone, King Acorn was forced to pledge loyalty to Ixis Naugus to survive. Over time, he lost his memory, not recognizing Sonic or Sally when they came to the Zone. He came to command an army of goblin-like creatures, clad in a barbaric suit of brownish-purple armor and claiming the Zone as his domain with an iron fist.
Later, the Freedom Fighters managed to free King Acorn from the Zone and brought him back to the safety of Knothole Village. Unfortunately, the King's body soon began to crystallize, apparently due to prolonged exposure to the Zone's harsh, alien environment (similar to the situation that occurred during SatAM). It was then discovered that the Sword of Acorns, when brought together with the Crown of Acorns and the King himself, possessed powerful healing properties and was able to restore King Acorn to his former self. It later became known that the wizard Ixis Naugus had been responsible for the King's condition, forcing Sonic to confront him and his minions-Warlord Kodos, Uma Arachnis, and Feist-in Max's dreams.
At one point, King Max was secretly replaced by one of Robotnik's Auto-Automatons, robots that are virtually indistinguishable from the living beings they're designed to imitate. While impersonating the King, the robot unilaterally sentences Sonic to life in prison in the Devil's Gulag prison when Sonic is mistakenly charged with the murder of Princess Sally. He also later brings Robotnik himself to Knothole, where Robotnik reveals that the King was a fake.
Following Robotnik's defeat, it came to light that King Max had been hiding the existence of other members of his family from Sally. These relatives were his wife Alicia and his son Elias. Shortly before Robotnik's takeover, the two of them had been sent into hiding, only to disappear and crash land on Angel Island. Found by the Brotherhood of Guardians, the young prince and comatose queen were cared for until they were eventually returned to Knothole. Alicia awoke shortly after this, but Max found himself paralyzed from the waist down. This necessitated his use of a wheelchair to get around. For unexplained reasons, he recovered from this condition during Sonic's Xorda induced, year-long exile in space.
Max led the Kingdom of Knothole in the first years of its ever raging quest to defeat the Eggman Empire. He was not well liked among the Freedom Fighters at first because of his arrogant decisions, which included disbanding them in favor of his own Secret Service (though he later had the Freedom Fighters reinstated), trying to force Sally into an arranged marriage with Antoine D'Coolette, and denying Sally her birthright. This is later revealed to be the result of his mind being half-crystallized, a side effect of time spent in the Zone of Silence.
King Max was poisoned by Patch, the Anti-Mobius version of Antoine, whom he was impersonating at the time due to an exchange brought about by Scourge the Hedgehog. The king slipped into a coma, forcing Prince Elias to take the throne to keep Patch from becoming King. Some time afterward, Max was revived by a device powered by a Chaos Emerald, though apparently confined to a wheelchair once more. He was among those captured during the destruction of Knothole and was beamed to the Egg Grape Chamber, only to be saved by the efforts of Sonic, Knuckles the Echidna, Tails, Amy Rose, and NICOLE, who saved the prisoners by having NICOLE transport them to the city of New Mobotropolis. When his old general Amadeus Prower rebelled against the monarchy, Max was outraged, denying Amadeus' criticism of the House of Acorn and extolling royal line despite the merits of Amadeus' accusations. Moreover, he used the opportunity-and Amadeus' short imprisonment-to try and plant mistrust of Sonic in Elias' mind. Influenced by his father, Elias ended up facing Amadeus in a duel, which was luckily brought to a halt by Sally, who settled the differences of the two and worked with them to create a new form of government. The new system of a ruling council in which the king served as a presiding councilor with six elected peers was received warmly by everyone-except for Max, of course, who became moody and refused to acknowledge his son after the news became public.
King Acorn also appeared in a future storyline where Sonic and Sally are married as King and Queen, and where he is a grandfather. This version of the future is where NICOLE originated. He does not appear in the Mobius: 25 Years Later future, possibly having died at some point between the present and the possible future.
Like many characters in the Archie Comics, King Acorn has varied alter egos in other dimensions. One version of him, Anti-Maximillian Acorn, also known as The Evil King, exists (but is never seen) on Anti-Mobius. This King Max was a complete despot, who was dethroned and cast into the Zone of Silence by his daughter Anti-Sally and her boyfriend Evil Sonic. Together they formed the Anti-Freedom Fighters and terrorized Anti-Mobius. Evil Sonic later mutated into a green hedgehog and reveals his real name as Scourge the Hedgehog and graduated to being a top-tier Sonic villain.