Earlier in the series he was shown to be a villain, but however recently in the series he has proven to be a valuable ally.
History and Origin
Sometime before the Mushroom War, Ice King was a normal, olive-skinned, dark-haired human named Simon Petrikov who was studying to be an antiquarian (a specialist in the knowledge of ancient artifacts). He had a young fiancée named Betty, whom he referred to as his "princess" and loved dearly. In "I Remember You," it is revealed that Simon was a Professor of Archaeology and had discovered the ancient text of heroes, the Enchiridion, during an expedition to the Hindu Kush mountain range. This is shown in a newspaper clipping that Marceline shows him in the episode "I Remember You." The newspaper heading reads, "Simon Petrikov Unearths Ancient Treasure," and was written by the journalist Bruce Guese. This newspaper clipping can be seen in an image above. At some point in his career, he purchased a strange jeweled crown from an old dock worker in Northern Scandinavia. After bringing the crown home, he put it on his head to make Betty laugh, but the crown caused him to black out and experience odd visions. He "fought with [the visions]... shouted at them until [he] realized it wasn't real—it was the crown." Simon had no recollection of what he had said or done while wearing the crown, though it was enough to frighten away his fiancée. Simon never heard or saw Betty ever again and began to record himself on VHS tape so people would know his story. As time passed, Simon would hear voices and continue to have disturbing visions even without the crown. Over time, his skin turned light blue, his nose became pointy and long, his eyes and hair turned white, he grew a full beard, and his body temperature dwindled to an unnatural low of 30 °C, or 86 °F (normal human body temperature is 37 °C or 98.6 °F). Despite these unnatural body conditions and being in a land covered by snow, Simon managed to survive. As his mind and body became further twisted by the crown, Simon thought himself "too far gone" to find an escape from its influence, though he hoped that he could one day regain his sanity enough so that Betty, his "princess," would come back and love him again. He is no longer considered a human; the power of the crown not only twisted his mind, but his body as well. His species/race is now Wizard according to the writers.Ice King was also Marceline's "beloved friend" and guardian before eventually abandoning her. Around the time of the Mushroom War and before losing his sanity, Simon found a young Marceline crying in the wreckage. He gave her a stuffed doll from a toy shop destroyed in the conflict to comfort her. This toy would eventually become Marceline's most precious possession, Hambo, which she would keep and cherish until it was sold to Maja the Sky Witch by her ex-boyfriend, Ash. Marceline described Hambo to Ash as her "favorite thing in the world." She was seen alone with a worn-out version of the stuffed toy when Finn and Jake visited her memories in "Memory of a Memory," possibly several years after Simon gave it to her. In "Sky Witch," Marceline is reunited with Hambo after Princess Bubblegum managed to swap it with Maja in exchange for the shirt Marceline gave her. It could be implied that the crown he wore only sought to protect him. It taught him the secrets of the "ice and snow," so he would survive the Mushroom War, as it did for Farmworld Finn in "Finn the Human." However, in the same episode, Farmworld Finn mentioned the crown's "promise of power," implying the crown's seductive nature and negative influence. As many years passed, Simon became disassociated from his original identity to the point that his only reaction to watching a video of his past was embarrassment at the idea of having once worn glasses. Further proof of his dissociation is shown in "What Have You Done?" when he responds to Jake's accusation of him stealing the crown with, "I didn't steal it. I made that item! Made it with the magic that I stole!" When others refer to him as "Simon" within earshot, he is generally confused or ignores it.
Though (in earlier episodes) Ice King was portrayed as disagreeable and short-tempered due to his main antagonistic role, he is merely a nuisance and "isn't usually a serious threat," according to Princess Bubblegum. In later episodes, due to no longer acting as a main antagonist, he is portrayed as more easy-going and having a sense of humor, particularly due to Ice King considering Finn and Jake his "friends" after the events of "What Have You Done?" The Ice King is misunderstood, lonely, and only hopes of marrying a princess. He has been labeled a "sociopath" by the Cosmic Owl, though his personality seems more akin to a person with narcissistic personality disorder, as he still feels humiliation and guilt. Due to his poor social skills, enduring short attention span and arrogant nature, Ice King is unable to attract any of his prisoners and often resorts to a forced marriage instead of the loving relationship he desires. However, he can be a loving and selfless person as revealed in "Princess Monster Wife." Aside from Gunter, his other penguin guards, and numerous ice creatures, Ice King has few other friends and expresses a continual zeal to spend time with anyone who will tolerate his company, especially his frenemies, Finn and Jake. In "The Eyes," he takes on the disguise of a horse to spy on Finn and Jake in an effort to learn how to be happy, but fails that as well. Despite Ice King's more antagonistic role, he plays the part of a protagonist in "Mortal Recoil," as he helps Finn and Jake defeat Princess Bubblegum by freezing her Lich-possessed body. In the mini-episode, "The Wand," he actually helps Finn and Jake regardless of the fact that Finn said to "sacrifice" himself, but unbeknownst of the fact that it also changed his face. Ice King is harmless in nature, and only kidnaps princesses and steals out of desperation. However, in the episode "Loyalty to the King," when he does have the opportunity to marry many princesses, he secretly plots to make an elite army of wives to take over the world with. His misunderstood intentions may either be in question or may just stem from his lunacy. His crown has made him insane over the years, affecting his social skills, causing him to unintentionally do bad things. Friendship and companionship is all he really wants, though goes about it the wrong way. Due to the insanity brought by wearing the crown, Ice King seems to have limited memory. This was first seen in "I Remember You," where he does not seem to remember his past as Simon Petrikov, even though he saw the history (after he found the crown) in "Holly Jolly Secrets Part II." Later on, he does not remember who Neptr is, even though he was the one that wanted Neptr to accept as his father in Neptr's debut, "What is Life?" and even confuses him with BMO, as seen in the events of "Mystery Dungeon." Ice King seems to find his happiness in his own Imagination Zone, and it is clear that he would rather live in his Imagination Zone than in the real world. In "Fionna and Cake," he is shown reading his gender-swapped fan fiction to Finn and Jake, based on the events and characters in the actual world. He is shown to cherish his fan fiction characters more than the real-life characters, and in "Mystery Dungeon" he tries to bring the characters to life, but he fails. In "Bad Little Boy," it is revealed that he had made ice sculptures of Fionna and Cake along with writing many other fan-fic stories. Although Ice King is characterized by these traits, Simon's personality is completely the opposite: he was known for his intelligence, kindness, and self-sacrifice. From the information given in "Holly Jolly Secrets Part II," Simon was a deeply committed and loving fiancé (to Betty). He proved to be the same way as a watchful guardian over young Marceline during the aftermath of the Mushroom War. Even when they were in great danger, Simon struggled to keep Marceline happy and helped her to always see the best of their situation. He took on a very responsible role in Marceline's life, by not only protecting her and keeping her healthy, but also by teaching her right from wrong and keeping her spirits high (as seen in "Simon & Marcy"). Simon, as a rule, was known for being optimistic even at the worst of times and it is clearly one of Simon's most notable attributes. Even while being saddled with a great burden, Simon fought an apocalyptic world while fighting the omnipotent influence of the Ice Crown. Some of Simon's innermost thoughts and fears of his inevitable madness were revealed in messages left to Marceline revealed in "I Remember You." Simon agonized over abandoning Marceline in a dangerous world due to his deteriorating sanity. However, Simon, nearing the very end of his humanity feared for his own life, worried that as the crown took him from Marceline, no one would be able to "save him." His vulnerability as a human was most evident just before he permanently became the Ice King.