The Abyssal Hunters consist of Ægir-Seaborn hybrid "super soldiers" created by the Ægirians as a countermeasure against the Seaborn, hence its name that derives from the Seaborn's origin. The process of creating Abyssal Hunters contains many different advanced procedures, and the candidates are chosen from among the whole Aegir population, regardless of their previous status or occupation.

Abyssal Hunters possess unusually high physical strength, endurance, and speed, far surpassing regular Terrans. Their physical and psychological engineering is necessary to withstand the Seaborn's might and horror. They exclusively use "primitive" melee weapons to emphasize their physical prowess as well as preventing their enemies from evolving countermeasures against their advanced technologies. Unfortunately, a hunter’s Seaborn blood can overtake their Ægir blood if left unchecked (as in the case of Skadi the Corrupting Heart), which can lead to their transformation into the monsters they despise. Symptoms of such transformation include lack of awareness for their surroundings, loss of consciousness, and Seaborn scales occupying their bodies.

Aside from their unimaginable strength and stamina, all known Abyssal Hunters possess white hair and red eyes, likely a sign of a successful merge with the Seaborn. However, Andreana, another hunter, lacks the traits listed above. And as such, is treated as a failed product; it is hinted that she might have been created by the notorious Church of the Deep. Abyssal Hunters are also entirely incapable of using Originium Arts, unlike normal Ægir people, as a result of their hybridization with the Seaborn.

Years ago, Aegir unleashed its entire military force to destroy the Seaborn Leviathans, especially targeting Ishar-mla the Firstborn.However, the price was unfathomable, as the Abyssal Hunters were almost completely annihilated. Those who managed to survive came to shore and have since been hiding their identities on land. And since then, the Church of the Deep has been attempting to kidnap hunters to conduct its cruel experiments.

Source: https://arknights.fandom.com/wiki/Abyssal_Hunters

Ulpianus: Ulpianus is the Third Commander of the Abyssal Hunters, being the oldest member of the group, who was thought to have died during the great assault on Ishar-mla many years ago. In fact, he not only survived the massacre, but uncovered the mystery of the Seaborn. He found himself inside an ancient ruin within the abyss where he witnessed numerous Seaborn "Genesezoics" and saw the moment Ishar-mla's blood fused together with Skadi's, turning her into Ishar-mla's vessel. Hence, Ulpianus remains cautious of Skadi and will use any method to prevent her from returning to her homeland out of fear she'd destroy it through Ishar-mla's revival.

Because of his familiarity with the Hunters, Ulpianus is well aware of their real secret as well as the Ægirian Counsels' corruption by the Church of the Deep. During the time he was thought to be dead, Ulpianus uncovered more secrets from the insidious Church after breaking into Cicero's laboratory. He learnt of the Church's intention to revive Ishar-mla and their ultimate goal of bringing the second Profound Silence onto all of Terra. Ulpianus also befriended the legendary Last Knight with whom he shares a common goal of defeating the source of the Seaborn.

Gladiia: Gladiia hails from a problematic family in which she faces various issues within, especially from her mother's. But despite the turmoil, she was able to become a high-ranking consul of Ægir's Honor Exercitus. She even volunteered to become a leader of the Abyssal Hunters to defend her homeland against the encroaching Seaborn. To Gladiia, not more is better that filling her troubled mind with Ægir's hollow pride.

Following the devastating battle against Ishar-mla and other Leviathans, Gladiia arrived at the shore and embarked on a search for the surviving Hunters. She eventually encountered Kal'tsit who holds such information and secretly worked for her and Rhodes Island. After the Sal Viento incident where she baited out one of the leading bishop of the Church of the Deep, Gladiia officially joined R.I. as a representative of Ægir and the nominal leader of the Abyssal Hunters. She has since been providing her cooperation with R.I. such as the hunt for the lost Stultifera Navis in order to re-establish the land's connection with the ocean. Despite the grim future that awaits her, she vows to accomplish her duty, return to her homeland, and defeat the Seaborn while racing against her subjugation to We Many.

Living under the ocean for most of her lifetime, Gladiia dislikes the surface's arid biome and often suffers from dehydration. Her haughty manner towards the land-dwellers, a consequence of Ægir's isolationism, causes others to be reluctant in interacting with her. Nevertheless, Gladiia proves herself to be a capable combatant utilizing a giant lance combined with her superior physical capabilities as a true Abyssal Hunter.

Specter: Once the daughter of an Aegirian architect who was forced to flee her homeland due to the Seaborn's invasion, the artistic girl named Laurentina voluntarily joined the Abyssal Hunters project that turned her into an Ægir-Seaborn hybrid supersoldier, which she believed to mold herself into her own perfect "masterpiece." However, after the devastating war against Ishar-mla, one of the Seaborn "gods," and her escape towards the shore, she was immediately kidnapped by Amaia, one of the leading bishops of the Church of the Deep, and became her experimental subject by being injected with a humongous amount of liquified Originium into her spine, enough to literally pollute an entire city with just a fraction of the amount. As a matter of fact, she contacted severe Oripathy in the process that led to her insanity, making her to behave as a psychopathic nun under the pseudonym "Specter."

Following a rescue mission by Kal'tsit, Laurentina was later brought to Rhodes Island to treat her illness. Even though her friend Skadi prefers her to stay back, she continues to take part in R.I.'s missions in order to look for prey to be lacerated with her buzz saw in combat. Her crazy laughter and her uncanny smile really unsettle both her allies and her enemies. On the other hand, there are signs that Laurentina could regain her sanity thanks to her Seaborn blood that resists Oripathy's influence. The day will come when Laurentina the artist will come back instead of Specter the nun.

Skadi: Skadi is an Ægir-Seaborn hybrid supersoldier who lost her family during childhood in a tragedy caused by the monsters. She was taken into the Abyssal Hunters project and was engineered to hunt down and exterminate the abyssal monsters. However, Skadi barely survived the devastating war against Ishar-mla and other Seaborn "gods" and luckily escaped to the shore. She has since disguised herself as a stoic bounty huntress known by the bizarre title "The Walking Catastrophe" thanks to her enormous strength as a Hunter. Fearing that her Seaborn blood would corrupt herself as well as bringing danger to others, she prefers to be alone most of the time, leading to her uneasy relationship with the land-dwellers. Nevertheless, Rhodes Island continues to employ her to help her investigate more about the current condition of the isolationist underwater nation and the surviving Hunters.

For now, Skadi uses everything she has to counter her Seaborn influence. But what if she fails? What if her Seaborn blood accidentally merged with Ishar-mla's during that war, turning her into His mere vassal in the process? What if she succumbs into the hivemind, causing her to slaughter her fellow Hunters to the last man? Will she ever become an apocalyptic disaster who leads the sea monsters to the land and destroy every civilization? Only Skadi herself knows the best.

Citations of the corresponding characters that are mentioned from above are depicted based from the source that has been taken upon the derivation and description of the said characters.

Ulpianus Bio : https://arknights.wiki.gg/wiki/Ulpianus
Gladiia's Bio : https://arknights.fandom.com/wiki/Gladiia/Story
Specter's Bio : https://arknights.fandom.com/wiki/Specter/Story
Skadi's Bio : https://arknights.fandom.com/wiki/Skadi/Story