Karkinos Rucan is one of the five Nodos, a powerful being contracted to serve the Silver Tribe in the anime Heroic Age. His name is derived from Karkinos, a crab from Greek mythology that assisted the Lernaean Hydra in its battle against Heracles, which directly informs the design and abilities of his Heroic Tribe form. Among the Nodos, he is the first to engage in combat with the protagonist, Age.

In terms of personality, Karkinos is characterized as a calm thinker who is not easily provoked or agitated. He maintains a level-headed and composed demeanor even in the face of conflict, setting him apart as a more contemplative and less impulsive warrior. A notable personal motivation is his deep care for his fellow Nodos, Yuty La. He expresses a genuine desire to protect her, stating on several occasions that he would do so even if his contractual labors with the Silver Tribe did not require it, suggesting a personal affection or attachment to her.

His role in the overarching story is defined by the contract he holds with the Silver Tribe, which consists of four distinct labors that he is bound to complete. As a Nodos, he is a significant force in the cosmic conflict between the various tribes, acting as an adversary and occasional ally to the human race. He is an adventurer who traveled to the Milky Way galaxy in search of a legendary artifact known as the Lamp of Aladdin. Outside of battle, he has simple personal tastes, as he is known to enjoy eating noodles and is depicted as a character who smokes.

The most notable transformation of Karkinos is his purple Heroic Tribe form called Lernaea. True to his mythological namesake, this form can further transform into a type of multi-headed hydra when on solid ground. The primary ability of Lernaea is initially believed to be control over Life, but it is later revealed to be the power of Restoration. His combat capabilities are immense, as he can employ a potent corrosive gas or material as a primary means of attack, and it is implied that this power is sufficient to destroy entire solar systems. His abilities include generating poisonous snakes and shields made of this corrosive gas, as well as manipulating matter to create miniature hydras or absorb it to heal and repair his body. Among his most formidable traits is his capacity for resurrection, making him an extremely persistent and difficult opponent to permanently defeat. When pushed into a desperate battle, for instance when fighting multiple other Nodos simultaneously, Karkinos can enter a dangerous berserker state known as Madness. In this state, his physical strength, speed, and durability are dramatically amplified, but he loses control and becomes unable to distinguish between friend and foe, becoming a destructive force that attacks indiscriminately.