Learn Italian: Ricordi la ragazza dai capelli biondi? - Do you remember the girl with the blond hair?

In Italian, when describing physical characteristics or distinctive features of a person or object, we use the preposition da combined with the definite article. So, dai capelli biondi means "characterized by the blond hair." While con i capelli biondi is understandable, using da is the more natural and descriptive way to say it.

Why is it dai and not just da?

Because it is an articulated preposition (a preposition merged with an article). You need to include the masculine plural definite article i for capelli (the hair). When da merges with i, it becomes dai.

Why is hair plural (capelli) here when in English we say "hair" as a singular word?

In English, "hair" is an uncountable collective noun, so it is singular. In Italian, hair on your head is treated as a plural noun, i capelli (literally "the hairs"). That is why the adjective biondi also has to be in the masculine plural form to match.

Could I say ti ricordi instead of just ricordi?