Sketchworld!Lux's tail acts as "storage tank" for extra light in case of emergencies, similar to how leopard geckos store fat. As a desperation move, he can discharge this light as a devastating beam using his stinger, which has a tiny crystal at the tip.
When he's been eating well, the tail is plump and squishy with a thick padding of fat. When he's absolutely STARVING and/or is running out of ink, it's emaciated-looking and somewhat stiff.
when he's upset, he starts waving it at you, kind of like this:
