Learn Portuguese (Portugal): O rapaz tornou-se um super-herói famoso.

We use tornar-se for a permanent, significant transformation, especially when someone turns into a new noun or identity (like becoming a superhero or a doctor). Ficar is used for temporary changes in emotion, health, or physical state, like becoming tired or sad.

Why is the pronoun -se attached to the end of the verb instead of before it?

In European Portuguese, the default position for object pronouns in an affirmative main sentence is after the verb, connected by a hyphen. So we say tornou-se instead of se tornou, which is much more common in Brazil.

If there were two boys, how would we make super-herói plural?

The plural is super-heróis. When a word starts with the prefix super-, only the second part of the word changes to match the plural. The accent on the o remains because it is still an open sound.

Can I say um famoso super-herói instead of um super-herói famoso?

You can, but it changes the tone slightly. Placing the adjective before the noun makes it sound more poetic or emphasizes the fame subjectively. Placing famoso after the noun, as in this sentence, is the standard, objective way to state a fact in Portuguese.