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?
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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.
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.)
The ideal of Shirou, the figure (of Shirou) who became a hero, is you.
Is the ideal of Shirou, the figure (of Shirou) who became a hero, you?
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.