sprite.setTransparency

sprite.setTransparency sets Transparency on a sprite object. It takes id and t.

sprite.setTransparency(id, t)

function start() {  const id = sprite.create('/sprites/hero.png', 100, 120);  sprite.setTransparency(id, 0.5);}

sprite.create is there to give the call something to act on; sprite.setTransparency is the line that matters. The code sits in start(), which the engine calls once, when the entity spawns.