Learn Spanish (Latin America): La mujer se había vuelto amargada y solitaria.

Volverse is used for profound, often permanent changes in a person's character, personality, or condition. Because becoming bitter (amargada) and lonely (solitaria) is a deep internal shift rather than a temporary mood (which would use ponerse) or a deliberate life choice like a profession (which usually takes hacerse), volverse is the best choice.

Could we use quedarse instead of volverse for this sentence?

Yes, but it subtly changes the meaning. Quedarse implies the change was the direct aftermath of a specific loss or event, as if she was left bitter and lonely after a tragedy. Volverse focuses more on the internal transformation she went through, which could have happened gradually over time.

Why is the pronoun se required before había vuelto?

Without the pronoun, the base verb volver simply means to return or come back. By using the pronominal form volverse, the meaning shifts to becoming or turning into something. In compound tenses like this past perfect, the pronoun must be placed immediately before the auxiliary verb haber.

Since amargada and solitaria are feminine to match la mujer, shouldn't vuelto also be feminine ()?