The idiomatic expressions that can be used to define Via's character in the novel Wonder should be centered on Via's character traits. Via comes across as a strong character who is very protective, even sometimes overprotective, of her brother, Auggie.
When looking at idioms, we must first know that these phrases or expressions typically have different meanings from what they literally say. Idioms are a form of figurative language and are therefore often used to provide imagery and personality to a piece of writing.
When looking for character traits to describe Via, you might first look at idioms that describe people. For example, the expression "worrywart" describes someone who often worries or has anxiety in different situations. On the other hand, the term "couch potato" is used to describe someone who is incredibly lazy.
Knowing that Via is very loving and kind toward Auggie might lead us to describe her as having a "heart of gold," meaning she is a good, kindhearted person and cares about others around her, especially her little brother. We might also describe her as being a "lone wolf," as there are many occurrences in Wonder where she keeps to herself and doesn't let others know when she may be struggling.