WARNING: The following contains spoilers for Season 5 of The Seven Deadly Sins, now streaming on Netflix.
Despite having the title of Fairy King, The Seven Deadly Sins member King noticeably lacked wings for most of the series. Finally, in Season 4, after returning from his trial trapped inside Fairy King Gloxinia’s body during the Holy War 3,000 years ago, King began to sprout wings of his own, giving him a boost in power. Gloxinia commented that King was on par with himself despite his wings having barely grown in terms of power. Now, in Season 5, he's able to spread them fully.
In Episode 6 of The Seven Deadly Sins' fifth season, "Confront Despair," King’s power is sealed during the battle with Mael. As he falls from the sky, no longer able to fly, he desperately forces power out of his body. Suddenly there is a flash of light, and Mael attacks Diane and Elizabeth. Before he can reach them, something smashes into him, sending him flying. The pair look up to realize it is King, but with fully grown wings and the appearance of a teenager, as opposed to that of a child.
Here, King embodies the shonen trope of the protagonist releasing a hidden power in a moment of need, like in Naruto, where Naruto could access Nine-Tailed Fox's chakra in moments of extreme duress. In many ways, King follows the path of a typical shonen protagonist, unlike Meliodas. Like many of the other characters in the Seven Deadly Sins, King’s motives are driven by love, specifically his love for Diane, and to a lesser extent, his sister Elaine. However, unlike Meliodas or even Ban, King also has the added responsibility of protecting his people, the fairies, as their king.
While his captain shirked the responsibility of becoming the Demon King, King does not shy away from the title of Fairy King. Instead, King’s responsibility often intimidates him, leaving him feeling unworthy of the title. King’s winglessness was a sign of weakness that, even after 700 years, he had yet to mature into a true Fairy King, unable to draw out the power of his spirit spear Chastiefol fully. But with this new evolution, he's has overcome this inadequacy, fully embracing his role as protector of his people.
Episode 7, "Hope, Conflict, and Despair," expands on this new form. Diane thinks to herself that King’s aura has completely changed, along with his outward appearance, and worries that his personality may have shifted as well. King turns and tells Diane not to worry, that he is still him, shocking Diane as she had not said the words aloud. King brings his attention to the mad archangel who suddenly attacks, destroying everything in his surroundings. After the dust clears, Diane and Elizabeth find themselves protected by Chastiefol.
Diane then calls out to King, worried that he'll be unable to fight while protecting her and Elizabeth. But as Mael attacks, the spirit spear’s guardian form strikes back at him, noticeably beefier than it had been before King’s evolution. As Mael prepares to retaliate, King attacks with the spear’s new form and once again with its original form. This is followed by a shot of King simultaneously wielding four of Chastiefol’s forms.
In addition to wielding multiple forms of Chastiefol at once, King has also displayed the ability to control the spirit spear remotely. In Episode 9, "That Which Gathers," King attacks the original demon using the spear over a great distance, although he comments doing so prevents him from using multiple forms of the spear.
This makes it clear that this is King finally realizing his true power. Gloxinia had mentioned that King was on equal footing with himself before his wings were fully sprouted. After they fully matured, King surpassed Gloxinia, accomplishing his dream of becoming a Fairy King capable of protecting his people.
Though he once viewed himself as an inadequate Fairy King, Season 5 of The Seven Deadly Sins has seen King fully realize his potential, becoming a power on par with the likes of Meliodas.