Maruruk is a Delver that lives in "Seeker Camp" of the 2nd layer of The Abyss alongside the legendary white whistle Ozen the Immovable. Despite his young age, he's already a Blue Whistle due to being a direct apprentice of a white whistle.
At some point when he was very young he fell down on an accident and nearly lost his life before being rescued by Ozen. Following the event, he chose to stay in the abyss and become Ozen's disciple but after some time his eyes developed an extreme sensitivity to the light, so he never returned to the surface. He's in charge of the telescope on the camp, and is often keeping watch of the surroundings in case a delver shows up so that a gondola can be lowered ahead of time.
Maruruk is a young boy with short blue hair and a rounded face. His feminine traits and maid attire leads people to mistake him for a girl, but he's actually male. While the details are not clear, it seems like Ozen was the one that forced him to dress like a maid.