Celsius

Prince Celsius is the half-brother of King Radius and the half-uncle of Stella, who abandoned Solaria after a huge dispute with Radius over who would be a better ruler.

Appearance
Celsius has a medium tan complexion, brown-grayish hair with sideburns and a Van Dyke beard.

He wears a purple turban with a white feather in the middle, a silk magenta coat with gold shoulder pads and trimmings; it has a purple belt. He has white puffy pants and purple high-boots with thick, gold outlines.

Personality
Celsius once had a caring and kind personality, but when he got older, he became conceited and strongly believed that he was the rightful ruler of Solaria. After an argument with his brother, he abandoned Solaria for his own selfish needs.

He is able to fake a humble demeanor as shown when he greeted his brother, and is shown to be very petty as he was willing to turn the people of Solaria against Radius at the cost of starting wars between Solaria and other kingdoms.

Season 6
In An Unforgettable Party, as the girls get comfortable and enjoy themselves, they catch a real-time news broadcast announcing the arrival of Prince Celsius, beloved brother to King Radius.

According to the former Solarian queen, Radius and Celsius grew up close, as if the two were fully related by blood, but Celsius believed himself to be a better, more appropriate fit for the role as king. This eventually sparked a quarrel within the royal family until Celsius ultimately abandoned Solaria, leaving Radius to continue on as successor to the throne. Since all of this happened well before Stella was born, it is natural that she remained unaware of his existence.

Just as Luna finishes her story, Celsius proclaims on TV that he realized Solaria was his one and only home after he spent years visiting various far-off worlds and realms. Believing that he and Radius should make peace, Celsius gestures over to a large chest he brought as a gift, but the one thing that catches Stella's attention is how he seems to be hiding his necklace. When Bloom asks what is wrong, Stella explains that all Solarians love things that shine, so there would be no point in trying to hide something like a shining jewel only to hide it. Bloom claims that she can just ask her half-uncle about it at the reception, which causes Stella to fly into a panic, as she completely forgot about it. Just then, a young servant notifies Stella of her dressing with her court ladies, but the Solarian princess insists that she can do it herself now that she is pressed for time.

Soon enough, the most anticipated guest, Prince Celsius, enters the ballroom and bows to his half-brother. Radius joyously accepts Celsius' homage, even voicing his relief at how his brother has finally recognized him as the rightful heir, however, a mysterious light overtakes Radius and causes the king to fly into a rage that he directs at Teredor, believing the King of Andros to be a monster attacking his daughter.

He demands that the guards attack this so-called monster but, when they express confusion over the situation, Radius snatches a halberd from one of them and charges right at Teredor. Stella quickly steps in front of both him and Niobe as she demands to know what her father is doing. As the whole crowd stands in shock, Bloom notices that Radius' eyes have changed and, once she recognizes what it is, quickly informs Stella that he was hypnotized. Using her light magic, Stella snaps her father out of his hypnosis and directs suspicion onto Celsius, believing him to be the sole culprit since he is the only one who would wish to sabotage the event. Having been found out, Celsius mockingly praises Stella for her sharp wit, then reveals that he planned to have the whole realm see Radius as a mad king, which would lead to him being ousted, even at the cost of triggering a realm-wide war.

Upon this revelation, Radius sends his guards after Celsius, who uses the light of his amulet to open up the chest he brought to the party. In doing so, he unleashes a swarm of mutant insects on the guests. This prompts both Sky and Thoren into action but, at the very moment Sky pulls out his Moon Khukuri, Celsius fires the same light at the pair of princes that he used on Radius. Bloom manages to strike the amulet, shattering it with a blast of fire, but she was too late, meaning that both Sky and Thoren are still left under Celsius' hypnosis.

Chaos continues to grow within the ballroom as the royal guards can no longer distinguish friend from foe, the party-goers continue to panic in their attempts to flee from the mutant insects, which have grown even larger. To ensure that no one gets hurt, the Winx will themselves into their Bloomix forms to guard the innocent and properly dispose of the insects.

Trivia

 * He is named after the word "Celsius," a temperature scale used by the International System of Units (SI).
 * Celsius' appearance and motives appear to have been modeled on Jafar from the Disney animated movie Aladdin.