Yoko Shiraki is a young woman born to the Shirakis, a very wealthy family who is known for their activities in the boxing field.She is the main female protagonist in Ashita no Joe. She is very polite, graceful and kind to other people, always acting exactly as expected from a daughter born to a family with a higher rank, donating to charities from time to time.As so, she clashes in both appearance and personality with most of the other characters who know nothing but poverty and hardship. One of those characters is Joe who is the first to remark that her so called 'kindness' is completely fake, implying her sympathy for the poor is merely a way to make herself feel superior over those who have nothing to eat.She is often very shocked by his actions but as time goes on she comes to be the one who understands him best. She is disliked by Joe as an 'evil woman' who tries to control his life by always having something to do with most the situations he finds himself in later in the series.
After gradually coming to like him(and showing very subtle and growing hints throughout the series, she confesses her love for Joe in the lockeroom right before the world title fight, begging him not to fight José Mendoza for her sake.