Pucca was an adopted superhumanThere isn't any mention of Uncles Dumpling, Ho, and Linguini having any siblings, and Pucca isn't exactly a normal human being since she can run pretty fast, mow through enemies threatening Garu with ease, and can take boulders to the head without any ill effects.
Pucca is actually a god in human form, or the god's Human CanIt would explain why she has no direct family, where she gets most of her physical abilities, how she suddenly gets super powers whenever the plot demands so, and why everyone but her supposed "enemies" seem to just naturally adore her. She's a God-Mode Sue who is actually a god! The show frequently claims that Garu is some sort of Chosen One, by this logic, Pucca is an avatar of a goddess sent to protect him.
Pucca is under a vow of silence as wellAt one point it's mentioned that Garu has taken a vow of silence until he has avenged his family. This explains while Garu is always silent, but not Pucca. It seems unlikely that Pucca is mute since in an episode where they were forming a band, it was mentioned that she "has a beautiful singing voice". So it's my theory that like Garu Pucca also took a vow of silence until she completed a goal. And considering how this series works, her vow is likely to remain silent until she makes Garu break his vow of silence by telling her he loves her. Course Garu being Garu this will inevitably take awhile.