Delvers

Delvers or called as cave raiders, is a general term to refer to adventurers who dive deep into the abyss to explore it in order to unveil its secrets. In the town at the edge of the abyss Orth, Delvers are incredibly highly regarded as national heroes and many children aspire to become one in the future. They delve in the hellish pit and return with invaluable artifacts hidden inside, exploring its depths and contributing to the study of the abnormal hole. Some orphanages like Belchero Orphanage teach orphans everything about delving like an academy, granting them the possibility of a bright future to those forsaken by life.

The greatest delvers are the ones known as white whistles, and they've made discoveries and retrieved relics that have changed the world. They're immensely admired by the population and considered the pinnacle of delvers. While many delvers work individually or in small groups, occassionally, official parties formed by the government in order to obtain certain items from the abyss. Because artifacts of a higher grade can alter the balance of power between countries, delvers from different nations sometimes compete to get their hands onto them.

Classification
Delvers are classified with whistles of different colors, based on their ability, experience and achievements.

Bells
Bells are the novices. Not a whistle at all. Given to those who are studying to become delvers, but have yet to actually go into the abyss themselves. Though most bells are given to young children, when Reg first arrived at Belchero Orphanage he was also given a bell until his first dive.

Red Whistles
Red Whistles are the apprentices. They are rookie delvers who have a decent knowledge of the abyss and have descended up to the first layer on their own. It is a common rank for prepubescent kids who have studied since an early age. One obtains a red whistle and the official title of delver after their first descent into the abyss. While they're fairly knowledgeable, red whistles lack the physical fortitude to get very deep, so they're only allowed to descend up to 550 meters. Going any deeper would be treated as an accident or a rebellious behavior, and a rescue party would be sent to retrieve them. If a red whistle somehow managed to descend 1350 meters, it is treated as suicide and search parties are called off.

Blue Whistles
Blue Whistles are the adepts. They are more experienced delvers who managed to prove themselves. Blue whistles are allowed to delve up to the second layer of the abyss, so they most likely possess an acceptable fighting ability to fend off the beasts of the forest of temptation. Typically and with the appropriate training, one attains the rank of blue whistle at the age of 15, though there are exceptions to this.

Moon Whistles
Moon Whistles are the teachers. They are highly proficient delvers that carry an extensive knowledge of the abyss. Moon Whistles can delve up to the third layer safely, so their delving ability is in a completely different level to Blue Whistles. They're considered experienced enough to serve as teachers for the newer generations.

Black Whistles
Black Whistles are the experts. They are extremely talented delvers that have mastered the techniques of delvers. Black Whistles can reach up to the fourth layer of the abyss, which is the point in which the strains of ascension can really make one touch insanity. They also sometimes work under white whistles as subordinates, and assist them in their expeditions.

White Whistles
White Whistles are the legends. They are considered to be the greatest delvers of all. Their achievements have changed the world and their discoveries astonished all, turning them into spectacular historical figures. White whistles are named after their unique persona and reach a whole new level incomparable to that of other delvers. They're armed to the teeth with artifacts of the abyss, and represent a formidable force. Because of their status, any information that is officially passed by them is considered a fact, no matter how absurd. White Whistles don't have any depth limit, however since the strains of ascension make it humanly impossible to survive the ascent from the 6th layer, for most of their life the 5th layer is their limit. At some point when they make the decision, a white whistle will descend to the inviolable 6th layer. Since return from this point is impossible, this descent is called the "Last Dive" and it represents the most important point of their career, delving into largely unknown territory.

The whistles themselves cannot be gotten through conventional means. White whistles are made out of Life Reverberating Stones, which can only be obtained through the remains of a loved one of the delver. For a Life Reverberating Stone to form, the deceased must love the delver unconditionally. Once the stone is refined into a white whistle, it will only make noise for the delver it was created for, and its sound can bring out the true purpose of any artifact.

Bells

 * Kiyui

Red Whistles

 * Natt
 * Shiggy
 * Reg

Blue Whistles

 * Maruruk

Moon Whistles

 * Jiruo
 * Zepo
 * Lerume

Black Whistles

 * Habolg
 * Torka
 * Tsumuredo
 * Guiera
 * Praying Hands

White Whistles

 * Lyza
 * Ozen
 * Bondrewd
 * Srajo
 * Wakuna
 * Riko

Trivia

 * Ironically, even though the White Whistle is the rarest and most difficult to obtain, more individual White Whistles are known by name than any other whistle classification.