20a star that gives no light
Estee trails a bright streak whenever it has to sprint back to you, which sells the idea that it lights the way. It does not. The pet emits nothing at all, so you still need torches underground and the glow is purely on the sprite.
buying it and the joke inside it
The Traveling Merchant brings it alongside the matching vanity set, so if you want the full look you take both in one visit or wait for his stock to come around again. The reference goes past the outfit: the pet is shaped like the collectible stars from the older Mario platformers, and dropping Reflective Obsidian Dye into the pet slot turns it into the grayed out version of a star you have already collected.