Mercury is a relatively shy planet and is easily intimidated. Due to him being the smallest planet in the Solar System, he is often the target of bullying by Venus, and occasionally, Earth. He is good-natured and careful with his words, as he tries to avoid hurting the feelings of the Sun when he asks if they are friends or not. He becomes more tolerant, standing up to Venus and the Sun.

Despite that, Mercury is overall a kind-hearted planet who slowly learns to stand up for himself.