The Sisters Grimm Wiki
Fairy Prince
Species: Everafter
Gender: Male
Age: Thousands of years old
Fairy Tale: A Midsummer Night's Dream
Oberon (father)
Titania (mother)
Puck (brother)
Sabrina Grimm (sister-in-law)
Alison Grimm (niece)
Emma Grimm (niece)
Skin color: White
Hair color: Blond
Status: Alive
Appearances: "Once Upon a Crime"

Mustardseed is Puck's younger brother. His parents are Titania and Oberon. In Once Upon a Crime, Puck gives him the throne to Faerie since it is generally agreed that Mustardseed would be a better king than Puck would. However, in the second epilogue, it is vaguely implied that Puck went on to take his rightful place on the throne of Faerie, therefore, again, implying that Mustardseed was a 'Regent King' of sorts.

Mustardseed is described as looking like Puck, except cleaner. Sabrina was surprised that Mustardseed was brothers with Puck, because the latter was grimy. He is also described at being more mature than Puck, although he is the younger of the two. He has blond hair, porcelain-like skin, and appears to be no older than eleven years.