Radius has orange-colored hair and eyes, a short boxed beard, and like most Solarians he has a slight tan complexion.
In season 1, Radius's civilian outfit is seen in Stella's nightmare. At the center of his golden crown is a smaller pattern of the crown in light orange. He wears a golden short breastplate with shoulder plates. Atop the breastplate is a collar plate that extends upward and encircles his neck area. An orange star pattern, shaped the like the Christmas star, is at the center of his breastplate. He wears a short, ivory caftan where its sleeves are long and tight, and is red on the upper parts of his sleeves. An orange cloth is tied to his waist, he wears red pants, and ivory knee-high flat boots where the soles are outlined with a tan border.
He wears a golden crown that resembles the sun, a red fuzzy hood, golden shoulder plates and a red cape. His sleeves are puffy with an ivory color, as the sleeves passes his elbow it becomes slender and gold. He has a collar shirt with a round golden necklace, belt, puffy pants and knee-high ivory-colored boots.
In season 8, he looks mostly the same, except his boots no longer have fur at the side, and his cape is a darker red.
He wears a blue robe, adjourned with yellow swirls and circles that appears to represent the sun on both sides of the robe. His sleeves are layered twice and lined with white ends; the robe's neck lining is white and puffy.
Although he could be somewhat absent-minded, arrogant and used to a life of deference like his daughter, Radius is a genuinely loving and caring father. He is also easy-going, warm and welcoming. Radius is quite fond of his Solarian doves and was upset when he found them caged up.
In season 5, he is shown to be stubborn and hotheaded. He dislikes the idea of needing someone's help as he turned down Luna's concern for him. His stubbornness can be seen when he disagrees in forming an alliance between kingdoms to divert Tritannus's threat. But if provided valid points and experience, he is willing to listen to reason such as when Stella got him to listen to her advice as she herself had faced Tritannus. It also appears that Radius's decision on joining the alliance is based on how well the kingdoms agree with each other.
Over time, he has calmed down and is much more cheerful. He was initially very bitter towards Luna but he slowly began to respect her opinions, no longer outright dismissing them and listens. He has also come to accept her presence in the castle and is seen smiling much more. As he is a ruler, he does get stressed from time to time.
As mentioned by Stella, Radius is not a party-planner and season 8 confirms this; it is also revealed that he does not know how to match colors. He loves ribbons and chocolate cake, dislikes lemon cake but enjoyed the flavor when the two were combined.
King Radius is first mentioned by Stella on the during the Magix holiday: the Day of the Rose (Earth's equivalent of Mother's Day). She, Musa and Bloom stayed behind and after Bloom explains why she did not go home to celebrate the holiday, Stella is then asked why she is not celebrating. That is when she tells them that her parents are divorcing so, there is nothing to celebrating.
Radius finally makes his first appearance in Stella's magically-induced nightmare. Him and Queen Luna are seen floating next to each other in front of Stella. Then, they begin glowing brightly, morphing into balls of energy and quickly spreading away from each other.
King Radius did not appear in season two, but was briefly mentioned by his daughter.
At first, all seemed well: he greets Stella with a loving paternal hug and tells Bloom to dispense with formalities, showing himself to be a genuinely caring father as well as an easy-going yet warmly welcoming host. However, things took a dark turn at the ball: he revealed that he was to marry Countess Cassandra, who proceeded to put both him and his daughter under a spell: Stella was transformed into a frog-like monster, and Radius' mind came to be completely under Cassandra's control. As a result, he failed to realize that Stella had been placed under an enchantment, and ordered his guards to arrest her, forcing the Winx to flee Solaria.
Later on, at Eraklyon's One Thousandth Year Anniversary Party, he was accompanied by Countess Cassandra and her daughter, Chimera, to the party. After the party erupts into chaos when Crown Prince Sky falls under Princess Diaspro's spell, Radius is caught in the battle and abandoned by Cassandra, literally left to the dragons. A dragon catches sight of the vulnerable king and approaches him, poised to attack. Upon seeing this, Stella selflessly rushes to her father's aid (despite his cries to her to save herself), pushing her powers far beyond their usual limits to protect him, resulting in her collapse.
Radius rushed to her, cradling her in his arms and pleaded with her continuously to respond to his cries, but as she remained unresponsive, he starts to cry. Fortunately, Stella's selfless act earns her Enchantix, and she regains consciousness in her new fairy form (much to the relief and pride of her father and friends). Father and daughter shared another warm embrace before she left with her friends.
Unfortunately for Radius, he once again fell under Cassandra's spell only this time it was no longer as strong as it had previously been, given his attempts to assert his will against her mind-control: he protested against her caging his doves and calling them "pests", begrudgingly agrees to banning Stella from attending their wedding, and later confesses that he was uncertain of exactly what he was doing. When the Winx learned of the wedding, they sabotaged it by revealing Cassandra's and Chimera's schemes to take over Solaria to everyone present at the event, and fought off the guards who tried to stop them. Stella uses her fairy dust to break Cassandra's spell over her father, and reuniting herself - joyfully - with her equally-relieved father.
King Radius appeared in "The Pillar of Light". Where he fell ill due to Tritannus taking the seal from the Pillar of Light. He also appears to have a harsher and more arrogant behavior, especially towards his ex-wife, Luna.
During the eclipse, he went to the room where Second Sun of Solaria was kept, in order to stay alright. Luna got worried about him and came to see him. Radius was not happy to see her, but before he could drive her away, he fainted and fell ill. Luna took him to his bed and took care of him. Stella got worried about Radius as well and was surprised to see her mother taking care of him. Out of worry, Stella leaves for the Infinite Ocean in hurry.
Stella soon manages to restore the Pillar of Light, but got another thing to worry: as Radius was alright, that would mean that her parents will start fighting again. At the throne room Radius and Luna welcomes Stella back. Stella tries to get her parents get along by saying that she thought of what both of them would have done and putted them together. Radius however ignores this, seeing what Stella means. Luna gets angry at him and they start fighting, resulting in Luna leaving the palace and Radius preparing to meet with Faragonda in anger.
Shortly after, Stella invites Radius and Luna to attend her fashion show. They react with enthusiasm and behind Stella still angry. In the Exhibition, Radius and Luna came closer together thanks to Stella's Sirenix wish so that the two aid in the final battles of the fifth season.
Appears in "Queen for a Day," when he promotes his own daughter as queen for one day. First, the Winx are allowed access to the Library of Solaria and then Stella leaves behind a gentlemen seeking help. After Stella abused his powers by creating a party queen and makes some changes to the guards, especially after Brandon went to speak with Stella, bothered by what happened, Radius can do nothing to ease his daughter's rage.
In a few moments, Radius is surprised to see that his daughter changes her attitude (due to the Magic Mirror) and brings five witches to attack her own friends and ordering the guards to let them fight. As Radius scolds her for her inappropriate behavior, the possessed Stella enchants him, preventing him to speak. He and Luna are protected by Bloom's shield while the Winx struggle with the witches, and Stella had to go to Legendarium after this incident.
After awhile, Stella had returned and he and Luna decide to see what she and her friends were doing in the castle's kitchen. He is shocked to see Stella working hard at making pizza but is proud nonetheless. Radius is then enjoying the party his daughter and her friends are throwing. The party is interrupted when a large beast crashes it. Radius is shocked at how it was able to enter his castle and comforts a scared Luna. After the Winx take care of the beast, Radius is given an appreciation speech by Stella, who eventually learned that being a queen is no easy feat, as it takes time, patience, humility, and most of all love. He is proud to see his daughter realize that as she returns his crown to him.
The Secret of the Lost Kingdom
He is briefly seen in the ballroom on Domino celebrating the revival of the planet, talking with his ex-wife, Luna, and his daughter, Stella. However, he does not dance with Luna. He has no speaking role there.
- The word "radius" means "ray" in Latin, and was probably the name given to Stella's father as reference to a ray of sunlight, Radius being shown to be associated with the Sun, proof that he cannot survive without the sun of Solaria if the Pillar of Light is damaged.
- His outfit since his first appearance as an illusion is similar with Oritel outfit.
- He, Ligea, Galatea and Miele were the four characters whom the Winx saved to earn their Enchantix.
- Interestingly, Radius is the only King who has suffered extensive damage in different seasons.