Learn Italian: Quell'attore ha un carisma incredibile, un che di magnetico. - That actor has incredible charisma, a certain magnetic something.

Un che di is a fixed expression used to describe an undefinable quality, similar to saying 'a certain something' in English. Here, che acts as a noun meaning 'something'. Just like with indefinite pronouns like qualcosa, you must use the preposition di before adding the adjective, and the adjective is always masculine singular (magnetico).

Could I just say qualcosa di magnetico instead?

Grammatically, yes! However, un che di adds a specific nuance. While qualcosa di magnetico just means 'something magnetic', un che di magnetico sounds more mysterious and intangible. It emphasizes that this magnetic quality is a subtle vibe or a hard-to-pinpoint trait.

Why is it un carisma? Doesn't the -a ending usually make words feminine?

Usually, yes, but carisma is a masculine noun. It belongs to a group of words derived from Greek ending in -ma that are masculine in Italian. Other common examples you have likely seen are il problema, il sistema, and il clima.

Why is it quell'attore with an apostrophe instead of quel attore?