The word that best pairs with "introvert" in relation to "shy" is "reserved."
Introverts are commonly described as individuals who tend to be more inward-focused, preferring solitude or smaller social interactions. Shyness, on the other hand, is characterized by discomfort or anxiety in social situations, often leading to hesitancy or reticence in interacting with others. While not all introverts are necessarily shy, the word "reserved" captures the idea of being more quiet, cautious, or restrained in social interactions, which aligns with both introversion and shyness.