Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. "Faithfully" describes how an action is performed (e.g., "He followed the instructions faithfully.").
* Adverbs often end in "-ly". While not all adverbs end in "-ly", many do, and "faithfully" fits this pattern.
Let me know if you have any other words you'd like to analyze!