The answer is that this is an application of a general rule of interpretation, taught in Maseches Megillah:
בְּמַתְנִיתָאתָּנָא: כׇּלשֶׁמַּעֲשָׂיווּמַעֲשֵׂהאֲבוֹתָיוסְתוּמִין, וּפָרַטלְךָהַכָּתוּבבְּאֶחָדמֵהֶןלְשֶׁבַח, כְּגוֹן: ״דְּבַרה׳אֲשֶׁרהָיָהאֶלצְפַנְיָהבֶּןכּוּשִׁיבֶןגְּדַלְיָה״—בְּיָדוּעַשֶׁהוּאצַדִּיקבֶּןצַדִּיק. וְכֹלשֶׁפָּרַטלְךָהַכָּתוּבבְּאֶחָדמֵהֶןלִגְנַאי, כְּגוֹן: ״וַיְהִיבַּחֹדֶשׁהַשְּׁבִיעִיבָּאיִשְׁמָעֵאלבֶּןנְתַנְיָהבֶּןאֱלִישָׁמָע״—בְּיָדוּעַשֶׁהוּארָשָׁעבֶּןרָשָׁע.
It was taught in a baraita: With regard to anyone whose actions and the actions of his ancestors are obscured and not explained, and the verse mentioned one of them favorably, for example, the way in which Zephaniah the prophet is introduced: “The word of the Lord which came to Zephaniah the son of Cushi, the son of Gedaliah” (Zephaniah 1:1), it is known that not only was he a righteous man, he was also the son of a righteous man. And conversely, whenever the verse mentioned one of them unfavorably, for example, in the verse that introduces Ishmael as the one who killed Gedaliah, which states: “And it came to pass in the seventh month that Ishmael the son of Nethaniah, the son of Elishama” (Jeremiah 41:1), it is known that not only was he a wicked man, he was also the son of a wicked man.
So since we don't know anything about Avichayil, and we know his daughter was righteous, he must be righteous too. This makes sense, because if his being her father was not relevant to the story, why would it be mentioned? It makes more sense that his character influenced Esther to be righteous herself.
