Esther was taken (Esther 2:8), ostensibly against her will, by king Achashveirosh. I would like to know if she had the halachic status of a captive such that, for example, the mitzvah of redeeming a captive (pidyon sh'vuyim)would apply.
Rambam (Matnos Aniyim 8:10, Meszler translation) seems to understand that a halachic "captive" must be one who is in danger:
פִּדְיוֹן שְׁבוּיִים קוֹדֵם לְפַרְנָסַת עֲנִיִּים וְלִכְסוּתָן. וְאֵין לְךָ מִצְוָה גְּדוֹלָה כְּפִדְיוֹן שְׁבוּיִים שֶׁהַשָּׁבוּי הֲרֵי הוּא בִּכְלַל הָרְעֵבִים וְהַצְּמֵאִים וַעֲרוּמִּים וְעוֹמֵד בְּסַכָּנַת נְפָשׁוֹת. וְהַמַּעֲלִים עֵינָיו מִפִּדְיוֹנוֹ הֲרֵי זֶה עוֹבֵר עַל (דברים טו ז) "לֹא תְאַמֵּץ אֶת לְבָבְךָ וְלֹא תִקְפֹּץ אֶת יָדְךָ" וְעַל (ויקרא יט טז) "לֹא תַעֲמֹד עַל דַּם רֵעֶךָ" וְעַל (ויקרא כה נג) "לֹא יִרְדֶּנּוּ בְּפֶרֶךְ לְעֵינֶיךָ". וּבִטֵּל מִצְוַת (דברים טו ח) (דברים טו יא) "פָתֹחַ תִּפְתַּח אֶת יָדְךָ לוֹ". וּמִצְוַת (ויקרא כה לו) "וְחֵי אָחִיךָ עִמָּךְ". (ויקרא יט יח) "וְאָהַבְתָּ לְרֵעֲךָ כָּמוֹךָ". (משלי כד יא) "וְהַצֵּל לְקֻחִים לַמָּוֶת" וְהַרְבֵּה דְּבָרִים כָּאֵלּוּ. וְאֵין לְךָ מִצְוָה רַבָּה כְּפִדְיוֹן שְׁבוּיִים:
The redemption of captives held for ransom takes precedence over sustaining the poor and clothing them. You do not find a mitzvah greater than the redemption of captives, for captivity is in the same category as famine, drought, or exposure, and one stands in danger to one's life. One who averts his eyes from redeeming [the captive] transgresses [the commandment], (Deut. 15:7) Do not harden your heart and shut your hand, and (Lev. 19:16) Do not stand upon the blood of your neighbor, and (Lev. 25:53) He shall not rule ruthlessly over him in your sight, and nullifies the commandment (Deut. 15:8) You must open your hand, and the commandment, (Lev. 25:36) Let him live by your side as your kinsman, and (Lev. 19:18) Love your fellow as yourself, and (Proverbs 24:11) If you refrained from rescuing those taken off to death, [those condemned to slaughter--if you say, "We knew nothing of it," surely He who fathoms hearts will discern], and many such sayings. You cannot find a greater mitzvah than the redemption of captives.
As a later posek, Aruch Hashulchan Yoreh De'ah 252:1 seems to follow this understanding as well. (Note that various Rishonim and Poskim disagree with this position, such as Tosfos, Pischei Teshuvah, and others.)
Megilas Esther (2:7) says Achashverosh loved her, and presumably he wouldn't hurt her1!
Therefore, even if she would have been considered halachically "captured" by Achashveirosh, it seems that her life wasn't endangered, so she would not meet the requirements for a "captive" according to the Rambam and Aruch Hashulchan.
1 on second thought, he slaughtered his previous wife, so perhaps she wasn't safe?
The gemarah ketubot 51b states as follows (with explanation from Sefaria.org)
ת"ר שבויי מלכות הרי הן כשבויין גנובי ליסטות אינן כשבויין והתניא איפכא
The Sages taught: With regard to women captured by the monarchy for the purpose of having intercourse with the king, they are considered to be like captives, i.e., they are assumed to have been raped but not to have consented to intercourse. However, those stolen by bandits are not considered to be like captives, as there is a concern that they might have consented to their captors, thinking that they will marry them. The Gemara raises a difficulty: But isn’t it taught in a baraita that the reverse is the case, i.e., women taken by the monarchy are not classified as captives, whereas this status does apply to those abducted by bandits?
מלכות אמלכות לא קשיא הא במלכות אחשורוש הא במלכות בן נצר
The Gemara answers: The apparent contradiction between the ruling of one baraita with regard to those captured by the monarchy and the ruling of the other baraita with regard to those captured by the monarchy is not difficult: This first baraita is referring to the monarchy of Ahasuerus, i.e., a powerful king, as the woman is aware that he is merely using her to satisfy his lust and will certainly not marry her, whereas that other baraita is dealing with the monarchy of ben Netzer, a man who established for himself a minor kingdom through robbery and small-scale conquests. It is possible for a woman to suppose that a king like ben Netzer will eventually marry her.
I would therefore conclude that Esther would have the halachic status of a captive and would be subject to the mitzvah of pidyon shevuyim.