|First Appearance||Big Sister Dora|
|Relatives||Dora & Isabella (sisters), Elena (mother), Miguel (father), Valerie (grandmother), Diego, Daisy and Alicia (cousins)|
|Voiced By|| Hans Alpizar, Vinicius Damasceno (Dora the Explorer)|
Miguel Cardona (Dora and Friends)
Guillermo Márquez is Dora's adorable baby brother. He is a 1 year old baby who has a pretty twin sister named Isabella. He wears a blue shirt and a white diaper. He appeared in many episodes of Dora the Explorer starting with "Big Sister Dora". Like Isabella, he remained unnamed until the episode "Dora's Jack-in-the-Box", where his name was revealed. Like Isabella, he appears in the Dora and Friends episode "Magic Land", in which he appears a little older and can speak in full sentences. They also helped Dora in Super Babies.
- In Dora and Friends: Into the City!, a slightly older Guillermo lives with his sisters, parents and Abuéla in the city of Playa Verde.