友 (yǒu) is a bound morpheme meaning "friend." Unlike the full word 朋友 (péngyou), 友 cannot stand alone and is typically combined with other characters to form compound words such as 女友 (girlfriend), 男友 (boyfriend), or 友好 (friendly).
她是我的室友,也是多年的好友。
tā shì wǒ de shìyǒu, yě shì duōnián de hǎoyǒu.
She is my roommate and also a good friend of many years.
他是一个值得信赖的战友和挚友。
tā shì yī gè zhíde xìnlài de zhànyǒu hé zhìyǒu.
He is a trustworthy comrade-in-arms and close friend.
校友会每年都会组织一次聚会。
xiàoyǒuhuì měinián dōu huì zǔzhī yī cì jùhuì.
The alumni association organizes a gathering every year.