Purely Capacitive AC - voltage and current relationship
Never mind numbers, just go with hand-waving.

Consider an initially uncharged capacitor. Now slam in a large current, that causes its voltage to rise. Now let the current decrease, and gradually become zero, this means the rate of rise of voltage will slow down and eventually the voltage will stop rising.

What I have just described there is the first quadrant of a sine wave voltage starting at zero, and a cosine wave current starting at max and falling to zero.

What the current does, the voltage does a quarter cycle later. The current leads the voltage.