An explanation of Jewish concepts is warranted prior to
answering the question itself.
When discussing Jewish religious concepts, the preferred
language is Hebrew as the Tanach (Jewish Bible) is in Hebrew. In
Hebrew, there is the title 'Tzadik', a Tzadik is a righteous
person. The root of 'tzadik' is 'tzedek' which means justice or
righteousness. Therefore, the title Tzadik is refers to a person
who seeks justice.
Sometimes, when discussing a subject with English speakers,
English words will be used. In this case, the title of 'saint' is
used in place of the word 'tzadik'. Obviously though, in this
context, the word 'saint' does not have the same meaning as the
Christian use of the word.
Now, to answer the question itself. Although Queen Esther is a
prophet in Judaism, she was not a tzadik. It was her uncle
Mordechai who was a tzadik.