Default Controls (remappable, or you can press Escape during keyboard set-up to keep them as is):

X: Jump, Double-Jump, Wall Climb (holding Left/right against a wall, while falling)

C: Attack (you can still move). There's a delay in-between attacks, but you can hold the button to fire again as soon as possible.

V: Bubble Shield (when available, its icon lights up)

Arrow Keys (Horizontal): Move...

Arrow Keys (Vertical): hold one of these and attack to use a special attack (their icons light up when available). Use the standard attack just before or after them to maximise your damage output.