Habolg is a well-known Black whistle, and was the person who delivered Lyza's White Whistle and final notes to the surface. He is called Habo-san by Riko and the other children.

Habolg is one of the delvers sent after Riko and Regu after they fled the orphanage and delved into the abyss alone. He caught up quickly, for the meantime, he give respects to Supreme Treasure of the Netherworld, Regu, because Natt and Shiggy insisted Habolg to see him. Instead of taking them back to Orth, he offered to escort them to the Seeker Camp, which by understanding, he then gave Riko a warning about Ozen, some medicine and other supplies, and let them go.

Since then, while he has not interacted directly with Riko or Regu, he has been shown on the surface telling stories to Nat and Shiggy about the abyss and its inhabitants, offering foreshadowing to the dangers that Regu and Riko will face.

(Source: Fandom Wiki)