FEMINIST is the one word substitute for:

One who works for the good of females

The word feminist is a one-word substitute for one who works for the good of females.

• One who hates females is a misandrist.
• Fashion diva is a person who is very interested in fashion.
• Female child is a girl.

The word "feminist" is the best fit because it conveys the idea of a person who believes in the social, political, and economic equality of women. The word comes from the Latin word "feminae," meaning "woman," and it was first used in the 18th century to refer to the movement for women's rights.