Sofia is strong, fierce, and daring to a fault. In fact, it is her refusal to lessen or belittle herself that almost leads to her destruction.
As a black American woman reared in the South in the 1930s, she rejects completely the systematic oppression that engulfed the position of the black woman. In that system, a black person had to remain absolutely subservient to whites — economically and socially. Blacks worked for whites, who paid them very little. In addition, a black woman came under the rule of her husband. A black woman was a virtual prisoner in the system. White men controlled the state, and black men controlled the black households. Sofia had no chance in such a setting. She simply wasn't suited for it by her very nature.
Sofia is younger than Celie, which partially explains why she is unable to accept the confining role laid out by the system for the "meek" black maid and the "dutiful" black wife. Sofia was meant to rule — not to take orders.
She fights back when Harpo tries to rule her with an iron fist. She wants a partnership relationship in their marriage, not a master-servant relationship. Sofia is a devoted mother and an excellent sister to her sisters, and she is generously forgiving. She even befriends Squeak, her husband's mistress, by volunteering to rear her child until Squeak is able to establish herself as a blues singer.
After Celie advises her stepson Harpo to beat Sofia into submission, Sofia confronts her. They are eventually reconciled, of course, but Sofia is thoroughly honest with others, as well as with herself. In contrast, when Celie admits — only to herself — that she wronged Sofia by telling Harpo to beat her, Sofia demands that Celie admit aloud that she told Harpo to beat her.
Although Sofia survives severe beatings during her imprisonment, she pays much too much for being herself, and in the process, she loses much of her strength and dignity. It is ironic that the value that she places on fighting back is the very thing that prevents her from living an independent life. Her adamant refusal to be a white woman's maid is eventually crushed, and she is forced to work — first, without pay in the prison, doing laundry, and then, with pay, as the white mayor's family maid. It is no wonder that she becomes a stranger to her own children. But it is to Harpo's credit that he loves Sofia more than anything, and he has a lasting love for her that proves that he respects her personhood.
In summary, Sofia is not tragic as much as she is symbolic of a woman who had the courage to fight against known odds.