I'm looking to clear up the use of で as:
動作を行うときの様態を表す。「急ぎ足で歩く」「親子同伴で出席する」「笑顔で答える」「土足で上がる」
or で as the て-form of だ with nouns/na-adj in this example.
describing a new girl the speaker just met
見たことのない制服で顔を真っ赤にしながら、口をわなわなとさせて目を真ん丸くする女の子。
見たことのない制服だ。顔を真っ赤にしながら、口をわなわなとさせて目を真ん丸くする女の子。- て-form of だ
or
[見たことのない制服で顔を真っ赤にしながら、口をわなわなとさせて目を真ん丸くする]女の子。- で as 動作を行うときの様態
Thank you.
I think this is the te-form of だ. If it's 様態, it means it modifies the following verb and describes how it is done. In other words, wearing a school uniform should be somehow important in the following main action (e.g., 新しい制服で学校に行く, 制服でゲームセンターに入ってはいけません).
However, in this case, none of the following verbs (顔を真っ赤にする, etc) is particularly related to her clothing, so I think this is a long relative clause that is describing two independent facts of the girl in parallel, i.e., "she is in school uniform" and "she had such-and-such a face".
Just to clarify, in Japanese, one can safely say その女の子は制服だ, although this sounds more or less colloquial (and even illogical in English).
Since uniforms are not 様態 but a thing, it should be a simple instrumental case, which represents that the girl makes such a face with the uniform.
Besides that, it could be the te form of だ too, while being a fairly colloquial sentence structure.
