Learn Hardcore Italian: Chi è il mio amico gentile?

Chi is specifically used to ask about a person. Che is a pronoun or adjective meaning "what," used to ask about things or to request a description of something rather than someone. Hence Chi is correct when asking "Who?".

Why is the article il used before mio amico gentile rather than leaving it out, as in English?

In Italian, possessive adjectives (like mio – "my") almost always require a definite article (such as il, la, i, le) before them when referring to singular or plural nouns, unless the noun is a singular family member in direct reference (e.g., mio padre). Since amico is not a direct family member, il is used.

Is there a reason gentile (the adjective) comes after amico (the noun)?

In most cases, Italian adjectives follow the noun. Some adjectives can come before the noun for emphasis or change of meaning, but gentile (meaning "kind") is commonly placed after the noun, so amico gentile is the natural word order.

Why does mio change to mio or mia, etc. in other contexts?

Italian possessive adjectives must agree in gender and number with the noun they modify. Here, amico is masculine singular, so the possessive is mio. If the noun were feminine singular (like amica), the possessive would be mia. For masculine plural (amici), it would be miei, and for feminine plural (amiche), it would be mie.

How would this question change if I wanted to ask "Who are my kind friends?"?

You would use the plural form for both amico and gentile to match the plural subject. So it becomes Chi sono i miei amici gentili?.