What is the possessive form of the name marites?

There are two accepted forms for possessive singular nouns ending in s:

Add an apostrophe (') after the existing s at the end of the Word: Marites'

Add an apostrophe s ('s) after the existing s at the end of the Word: Marites's

Examples:

Marites' brother helped her with her homework.

Marites's brother helped her with her homework.

Note: The name of a person is a proper noun. A proper noun is always capitalized.