The thing that’s funny and relatable about Luigi isn’t that he’s a coward, it’s that he’s a coward with no sense of scale. He reacts to literally everything with exactly the same level of oh-gee-I-dunno apprehension, whether it’s hearing a noise in a dark house at night, or coming face to face with a world-devouring horror. This is more or less the root of his self-esteem problems; he’s the universe’s second-greatest warrior and doesn’t realise it because as far as he’s concerned, being scared of a spider in the bathroom and being scared of Cthulhu are the same thing – he just totally lacks the necessary context to appreciate that managing to grit your teeth and deal with the second one is any sort of achievement. It’s all spiders in the bathroom to him. He’s equally terrified of everything, and that makes him invincible.