Do you want an origami ninja star with んです

Grammar Explanation: Reasoning with 〜のだ

A couple of girls admire the foreign student Sotoka, as she has a sparkle about her. Sotoka notices the girls, mistaking their admiration as being for the origami ninja stars adorning her school bag.

ソトカ:: 「ほしいんですか？手裏剣(これ)」; “Is it that you want one? This.”; Literal: “Is it that you want it? This.”

Key Points

1ほしいんですか asks for the reason behind what Sotoka sees

んですか asks for an explanation or confirmation, not just a bare yes-or-no fact.
Sotoka sees the girls looking at her and guesses the reason is that they want the origami ninja star.

2ほしい describes wanting a thing

ほしい is used when the desired object is something you want to have.
Here the object is 手裏剣(これ), the origami ninja star on her bag.

3手裏剣(これ) works as a clarification after the question

The main question is already complete as ほしいんですか.
Sotoka adds 手裏剣(これ) afterward to clarify what she thinks they might want.
The furigana これ shows the spoken word is “this”, while the kanji 手裏剣(しゅりけん) identifies the thing she means: the origami ninja star.