他

他 is a Personal Pronoun, which means “he or him”.

Usually, 的 is not used after a personal pronoun, or 谁, an interrogative pronoun referring to a person, is used attributively to modify a noun of kinship or institution. For example:

我 / 你 / 他 / 谁 + (的) + 亲属（爸爸 / 妈妈 / 哥哥 / 姐姐……）

我的爸爸 = 我爸爸（OK）

你的妈妈 = 你妈妈（OK）

他的哥哥 = 他哥哥（OK）

谁的姐姐 = 谁姐姐（OK）

However, when they express possession, 的 is obligatory. For example:

这是他的汉语书吗？

这是谁的衣服？

Note: The gender of personal pronouns

In written Chinese 他 refers to a male person and 她 or 她们 female, but the difference is lost when spoken because of their same pronunciation.

他们 generally refers to the male but may also include the female. e.g.

1) 他们（哥哥和妹妹）回来了。They (brother and sister) came back.
2) 张老师教他们。（男同学和女同学）。Miss Zhang teaches them. (Male and female).