Sally is a sad character in The House on Mango Street. She is beautiful and has "eyes like Egypt," and she attracts a great deal of attention. Her father is strict and thinks that being beautiful is trouble, so he makes her come right home after school. Though Sally has a definite style in public and wears black suede shoes and a black coat that make the narrator jealous, she has to remove all her makeup and what makes her outwardly distinctive when she goes home. Her friend, Cheryl, has dropped her, after an incident in which Sally made Cheryl's ear bleed, and Sally is alone. She leans against the fence in the schoolyard alone, and the boys circulate stories about her that aren't true. Sally is portrayed as lonely and only wanting love. Her beauty and distinctiveness have caused her to be isolated rather than loved.

I'm going to take your question in two parts.  The first one is this:  what techniques does the author use to describe Sally?  The answer to this is:  the author uses the character's words and actions and, more importantly, the narrator's reactions and thoughts to reveal the character of Sally.

The second part is this:  what is Sally like?  Sally appears to be a confident and outspoken girl, who wears flashy clothes and doesn't hold back from speaking her mind.  Although Esperanza looks up to her at first, she realizes that Sally's life has been troubled and that Sally herself is just trying to get through the pain of it. 

What happens to Sally in The House on Mango Street?

Sally in The House on Mango Street is a beautiful girl Esperanza admires. She has "eyes like Egypt" and long, shining black hair "like raven feathers" that lays on her back like a "satin shawl." From her, Esperanza learns how to wear mascara. She envies Sally her black clothes and her nylons.

However, Sally has a sad life. Her father is a tyrant who fears her good looks will get her into trouble. Sally has to hurry home after school and can't come outside again. Nevertheless, she has a reputation among the boys for being promiscuous. Esperanza doesn't want to believe this is true.

Sally ends up running away and marrying young to escape her abusive father. She marries a marshmallow salesman and says she is happy in her nice new apartment with her toaster and linoleum floor. However, she has traded one tyrant for another. Like her father, her husband has fits of rage and forces her to stay home. She has to sneak out when he is not around.

Sally represents a free spirit who wants to enjoy her beauty and sexuality without being shamed for it. She also represents the sharp limitations that face a poor woman who tries to live a conventional life of marriage and dependency on a man. Her potential is wasted before she is old enough to fully understand what she has done.