士郎の理想、英雄となった姿があなたではないのですか。
I think the subject of なった is 姿, but a friend tells me it's 士郎.
Is it not possible that the sentence means
The figure that became hero, are not you?
?
士郎の理想、英雄となった姿があなたではないのですか。
I think the subject of なった is 姿, but a friend tells me it's 士郎.
Is it not possible that the sentence means
The figure that became hero, are not you?
?
As a whole sentence, 「士郎の理想、英雄となった姿」 is the long subject phrase. If I have to narrow down, 理想 and 姿 are the two parallel subjects.
According to this Wikipedia article, this tweet, and this page, this question is made in a special context. Here, the speaker is talking to Archer, who is supposed to be the reincarnation of Shirou, who wanted to became a hero. Archer says he and Shirou are two separate beings. But the speaker believes that the person in front of the speaker, Archer, and Shiro are virtually the same person.
- あなたが士郎の理想です。
You are the ideal of Shirou.- 士郎の理想があなたです。("you" emphasized in this form)
The ideal of Shirou is you. (or It's you that is the ideal of Shirou.)- 士郎の理想、英雄となった姿があなたです。
The ideal of Shirou, the figure (of Shirou) who became a hero, is you.- 士郎の理想、英雄となった姿があなたですか。
Is the ideal of Shirou, the figure (of Shirou) who became a hero, you?- 士郎の理想、英雄となった姿があなたではないのですか。
Isn't the ideal of Shirou, the figure (of Shirou) who became a hero, you?
In the first half of the sentnece, there is a relative clause "英雄となった". And the subject of the なった is 士郎, as your friend suspected. You can understand it like 「士郎の理想、(つまり/そして、)士郎が英雄となった姿」 (Here's the article about GA-NO conversion, just in case you don't know that)
AがBとなった姿 = the figure of A who became B; A in the form of B; etc.
英雄となった姿 is followed by が which should tell you it's not that. Ex: 彼は目が青。(subj: him).
Can't tell w/o some reference, but the subject is あなた. Could be Shirou, might not be.