I cover your head, but I am not a hat. I am often attached to a coat. What am I?

I am your largest organ, and first line of defence, I have three different layers but the top one you know best, It keeps out water and cold out, and melanin defines its colour, It's not just humans who have me; there's a lot of things I can cover

I may keep a fence standing, or keep your friends updated.

I have a head but never think, I rule the court but never blink. Clothed in gold or silver bright, I am the symbol of royal might. Held in the hand of queen or king, I do not speak, yet my presence sings.

I keep things in; I keep things out. A click or clack means I’m about. A memory of one’s lost past; A haircut, maybe first or last. These two things share a name alike, And don’t forget one for your bike!