Koenma typically appears as a toddler with a pacifier in his mouth, wearing a blue robe with a red obi sash, and a large hat with the kanji for "king" and "Jr." However, he can transform into a young adult form when dealing with matters outside the Spirit World, retaining his pacifier and the "Jr." mark on his forehead in this form.

Koenma is known for his brattish, spoiled, yet hardworking nature. He loves food and can be easily distracted, but he's also shown to be caring, loyal, and strategic when needed. His character provides both comic relief and serious leadership moments throughout the series.

While Koenma isn't a combatant like other characters, he possesses significant spirit energy. His pacifier is notably one of the most powerful items in the Spirit World, capable of creating a powerful barrier or being used as a weapon. He uses this energy to maintain his adult form, which requires concentration and energy.

Koenma has been featured in various merchandise, fan art, and is a beloved character for his unique design and dual nature. His interactions, especially with Yusuke, provide comedic and pivotal moments in the narrative.