So I am in chapter 4 of this textbook I am using to learn Latin, and this is the first time I have ever seen declensions being used for proper nouns. I could use some guidance on when and why I should be using declensions for proper nouns, since I have not seen them until just now. Wish the textbook gave some context. Nevertheless I will give some context from the textbook.

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Sextus est puer molestus qui semper Corneliam vexat.

(Sextus is a troublesome boy who is always annoying Cornelia.)

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and

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Cornelia igitur Sextum non amat.

(Cornelia therefor, doesn't like Sextus.)

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Why are these proper nouns being Pluralized? Is it because of the accusative nature of the sentence involving the proper noun?