It makes sense for Glover's character to appear in the spinoff. After all, he first appeared in Episode 9 of Adventure Time's third season, suitably titled "Fionna and Cake", as a gender-swapped version of Marceline.
Oddly enough, when Marshall Lee appeared in Fiona and Cake, he appeared as a simple young adult human who was a friend to Fionna and a local street musician. This is obviously quite the shift from the usual half-demon vampire king that fans are used to seeing.
Donald Glover's Surprise Return for Fionna and Cake
In an interview with Inverse, Fionna and Cake showrunner Adam Muto spoke about Donald Glover's big return as Marshall Lee in the new animated Adventure Time spin-off.
Muto revealed that the actor "was just game to do it," which "was a surprise" to the showrunner:
"He was just game to do it. They were all established voices, so we just went back to everybody and saw if they were interested, but he was a surprise. I figured he would be the busiest out of everybody, but he really made time to be part of the show, and I appreciate it. I think he just does whatever he’s interested in. So it was really heartening that he would come back."
When asked if Glover was involved in writing any of Lee's new songs, the showrunner admitted that he was not, but if he was, "that would've been like another year of negotiating:"
"No. I mean, we had to get permission from his label and all that, but that would've been even harder. If I approached him and said, 'Hey, can you write a song for us?' That would've been like another year of negotiating.
Muto shared that the song was "co-written by Patrick McHale and Somvilay Xayaphone" and it was a product of their "fake band:"
"That was a song co-written by Patrick McHale and Somvilay Xayaphone that they had from years and years ago. They had a fake band with Pendleton Ward they had written a couple of songs. I think it was called The Bloody, and that was one of the songs they wrote for it. I could be conflating the fake bands they came up with over time."
Not Every Original Voice Actor Returned With Donald Glover
It's easy to understand why some would be surprised that Donald Glover returned in the first place. He is a very busy actor, writer, producer, and musician, after all.
Sadly, not everyone has returned to their roles in Fionna and Cake. Most notably, Kumail Nanjiani recently made headlines when he revealed that, to his knowledge, he was never asked to reprise his role as the god Prismo.
According to the new Adventure Time spinoff's creative team, Nanjiani's reps were sent casting offers from them, but it seems that they never made their way to the actor himself—it's a whole thing.
Another original cast member who didn't crossover was Neil Patrick Harris as Prince Gumball.
This isn't the first time Donald Glover has made an unexpected return to a notable role. Most recently, he shocked fans when he reprised his role of The Prowler in a live-action cameo for Spider-Man: Across the Spider-Verse.
Fionna and Cake is now streaming on Max.