Not to be confused with the Venezuelan general of the same name
He gets his fur color more from his father but has his mother's eyes. Because he is five years old about he wears normal clothing but does not wear a nappy, his clothing consists of dark blue trousers and a black and white striped T-Shirt
Guillermo may be five but he acts like a baby and is also sometimes treated like a baby and is a baby when Fructuoso introduces his children to Leopoldo as he is just two years old. Interestingly enough his behaviour is not only similar to a human baby but also the Junta babies. Even though he acts like a baby he is not as frustrating to the older Junta children as the Junta babies are.
He is the closest to his mother alongside Isabella and has a mischievous streak in his personality but like his sister he is phonophobic but he is not startled when Heinrich pushes over a lamp in a rage. Instead he is more startled when he is heard shouting but despite the fact he shouts on stage Guillermo knows he is acting and instead of being startled he is amused especially at his acting and although he enjoys singing his age (5 years old) makes him play "Oliver" for "Pick a Pocket or Two". Like his siblings Guillermo also has a curious streak and like Basilio when Leopoldo is told to watch him he often tries to sneak away and most of the time succeeds. He is also shown to be the most concerned for Cristina after she is captured in the early days of the Junta of Animalia and is the second most emotional when she is brought to Argentina.
Despite the fact he is older than a baby's age he has the gait and walk of a baby unlike his siblings who have the gait of normal children. He not only has friendships with other young children but also has friendships with the Junta babies and unlike his siblings is relatively close to them.