Here's why:
* Proper nouns are names of specific people, places, or things.
* Allie is likely a name, making it a proper noun.
If "Allie" was used in a different context, it could have a different part of speech. For example, "Allie" could be a verb if it was used as a slang term for "allied" or "allied with". However, without more context, we can assume "Allie" is a proper noun.