Ursula is a bubbly, flighty, mostly-lesbian mermaid just looking to have a good time. It's not her fault if her idea of a good time involves luring people with siren song and drowning them. It's just in her nature.
In her world, modern Earth, gods and creatures of myth/legend/folklore exist and have power proportional to the amount of belief in them. They are strong, powerful, immortal... and desperately vulnerable.