Seb is a special episode character in Candy Crush Saga. He is a musician is featured for the first time in the 468th episode, Creator's Crib. Before introducing level 6996, he and Tiffi had a dance party in the living room.
Seb was actually based around a real person, and he has passed away around the time this episode was released. He was a level designer at King that worked with us for around 5 years and his contributions to this game were amazing, bringing to the game some many new levels throughout all that time. The 7000th level is created in memory of him. 
He is chronologically the last episode character released, and was created after Licorise, one of the two characters created specifically for the rotating textures.
It also happens Casey will appear on the next episode after Seb's episode.
Creator's Crib is a rare instance of an episode containing a completely new texture and character at this current time of new releases.
Before the new texture, the original name of Episode 468 was Choco City with the champion title, Dandy Detective. While nothing is special with level 7,000 itself, the new episode texture and character featured is most likely a celebration for another milestone in Candy Crush.
He is the last character to ever be introduced in an official episode.
His only appearance is in the one non-tutorial episode exempt from the April Fool's Day (2020) episode names and backgrounds at the time of release.