Martlet is the deuteragonist of Undertale Yellow, the boss of Snowdin, and the final boss of the Genocide Route. She is a member of the Underground Royal Guard.

Martlet is a blue anthropomorphic bird with two wings for arms. She wears two brown boots over her feet, with a cotton trim. She has a mop of hair on top of her head and wears dark blue pants.
When Martlet transforms with her serum, her eyes turn a glowing yellow and her hair, wings, and legs become longer.