Being a Royal Guard, Martlet is very familiar with the propaganda that human beings are essentially evil creatures. Regardless, she is remains skeptical of the propaganda, firm in the belief that there is good in everyone. This leads her to be overtly kind and diplomatic with Clover in almost every circumstance. However, should Clover be utterly merciless in his approach to monsters, Martlet will be forced to face the reality that the only way to stop Clover is to kill him. From the moment she met him, Martlet had a backup plan in case Clover were to be as evil as the propaganda claimed - a vial of Determination extracted from a human Soul. On a No Mercy run, she injects it into herself, resulting in her transformation into Zenith Martlet.

Clover eventually destroys her breastplate, causing her wings lose both their cosmic luster and their flight abilities. Despite this "weakened" state, she is as strong as ever, and remains determined to end Clover's rampage. As the fight wears on, however, Martlet begins to increasingly suffer from the effects of too much Determination in a monster - her body begins to drip and melt. Upon her defeat, her body completely melts down in horrifying fashion, leaving her screaming in fear and agony before she is scattered into dust.