Curate, connect, and discover
Severus Snape and Sirius Black are narrative mirrors, mainly for their shared grief for a lost loved one but also for how they both projected their relationship with James onto Harry.
Sirius saw James in Harry. He wished James was in Harry and the main reason he protected and loved Harry was because of his devotion and grief for James. He did love Harry regardless, but he still connected James to Harry significantly to a point he didn’t recognize Harry as his own person.
Severus saw James in Harry. He convinced himself James is in Harry, because he hated seeing Lily in Harry. He hated looking into those eyes, so he kept himself in denial, because that's how he coped.
Sirius isn't a manchild that only saw Harry as James. But there is a character in canon that is like that, and that character is Snape.