hers

We don’t traditionally think of hers as having synonyms, but there are several situations in which it can replace—or be replaced with—another term.

her

Sometimes you can reword what you’re saying to use her instead of hers. For example, instead of saying This seat is hers, you could say This is her seat. 

her own 

The phrase her own is sometimes used as another way of saying hers. For example, instead of saying After tasks are assigned, Kate should focus only on hers, you could say After tasks are assigned, Kate should focus only on her own.

theirs

Instead of saying things like This house is his and hers, we usually use the plural form theirs, as in This house is theirs. Sometimes, though, it makes more sense to separate who owns what by using separate possessive words. For example, instead of saying These vehicles are theirs, you could say This car is his and this truck is hers. 

They has a long history of being used to refer to an unspecified individual or to a person whose gender and other personal details are unknown or irrelevant, and theirs can also be used as a singular in this way, as in I’m not sure who that is, but I think this car is theirs. Sentences like this sometimes use constructions like his or hers, but the use of theirs has become very common in such instances, with many people preferring theirs as less awkward and more inclusive.

Strictly speaking, hers doesn’t have a definitive antonym in the same sense that short is the antonym of long. Of course, possession and association are often distinguished by using contrasting possessives, such as in statements like This responsibility is hers—it’s not mine. In this sense, all of the other possessive words could be considered the opposite of hers, including mine, his, theirs, and ours.

On the other hand, one person’s possession of something does not mean that someone else can’t possess it as well. This is indicated in expressions like what’s mine is hers.

Hers is a pronoun. It’s a form of the possessive case of the personal pronoun she that’s used as a predicate adjective (a type of adjective that modifies or describes the subject of a sentence or clause and is linked to the subject by a linking verb, such as a form of the verb be), as in Those books are hers. It can also be used as a pronoun that functions as the subject, as in Hers are the red ones. 

Like the similar possessive words mine, yours, his, and ours, hers stands alone and never accompanies another noun as a modifier.

In contrast, the word his can be used as both a possessive pronoun (as in That jacket is his) or a possessive adjective (as in That is his jacket).

Hers is used along with third person pronouns (like she and her), which refer to anyone other than the person doing the addressing or the ones being addressed. Other third person possessive forms include their, his, her, and hers. In contrast, first person pronouns (like I, me, we, and us) refer to the speaker or writer themselves. First person possessive forms include my, mine, our, and ours. Second person pronouns (like you and yourself) are used to address the person being spoken to or reading. Possessive forms of these include your and yours.