Learn Spanish (Latin America): Es mi único hermano.

Usually, descriptive adjectives go after the noun in Spanish. However, único changes meaning depending on where it is placed. Before the noun, it means "only" (as in being the sole one). If it went after the noun (mi hermano único), it would mean "my unique brother."

Why doesn't the sentence start with Él for "He"?

In Spanish, subject pronouns like él (he) are often dropped because the verb form es (is) and the context make it clear who you are talking about. You could say Él es mi único hermano, but leaving the pronoun out is much more natural.

Does único change depending on who I am talking about?

Yes! Because it acts as an adjective here, it has to match the noun it describes in gender and number. If you were talking about your only sister, you would say mi única hermana. If you were talking about your only siblings, it would be mis únicos hermanos.

Since we are learning about superlatives, can I say el más único to mean "the most unique"?

Generally, no. Just like in English, something is either unique or it isn't, so saying el más único (the most unique) sounds unnatural to most native speakers. It is better to just use único by itself.