Astral is a mysterious energy being who originates from a parallel dimension known as the Astral World. He serves as the secondary protagonist of Yu-Gi-Oh! ZEXAL II, appearing as the spiritual partner and closest friend of the main hero, Yuma Tsukumo. Upon his initial arrival in Yuma's world, Astral suffered from complete amnesia, having lost his memories and his purpose. These memories were scattered across the human world in the form of one hundred Number cards, powerful and often dangerous monsters that possess their users. Astral understands that by confronting and defeating the owners of these Numbers in Duels, he can absorb the cards and gradually restore his fragmented past. This quest forms the core of his partnership with Yuma, who is one of the few people capable of seeing and communicating with him. Because he is usually invisible to others, Astral is often referred to by Yuma's friends as the Dueling Ghost.

Astral's personality is marked by a sharp contrast between his analytical brilliance and his profound naivety about the human world. In the context of a Duel, he is exceptionally intelligent, level-headed, and strategic, able to remain calm and devise complex tactics even in the most disastrous situations. His vast knowledge of the game allows him to quickly identify the weaknesses in an opponent's strategy. However, outside of Duels, his amnesia and alien nature make him incredibly curious and literal-minded. He approaches simple human activities, such as eating, using a bathroom, or watching television, with a sense of genuine wonder and often takes metaphorical statements at face value. This curiosity frequently manifests in his habit of making numbered observations, where he announces his analysis of a new experience or situation, such as Observation Number One, Two, or Three. Despite his typically serious and stoic demeanor, he is not without emotion. Astral develops a strong fear of the rival duelist Kite Tenjo after an early defeat, which he must consciously overcome. He also harbors an unexplained fear of cats. As the series progresses, he learns to smile more often, a change that reflects his growing emotional connection to Yuma and the human world.

Astral's primary motivation throughout ZEXAL II is the recovery of his lost memories, which are directly tied to his mission from the Astral World. While he initially forgets this mission, he is driven by a fundamental need to reclaim his identity and understand why he was sent to Yuma's dimension. As he collects more Numbers, he gains glimpses of his past, including visions of his own birth and his home world under attack. Beyond this personal goal, his role in the story evolves to become Yuma's guide, conscience, and strategic partner. He constantly advises Yuma during Duels, although Yuma's impulsive nature often leads him to ignore this advice with mixed results. Astral is intrinsically linked to the Numbers; he states that he is their embodiment, and his very existence is tied to them. If the Numbers are forcibly taken from Yuma, or if Yuma loses a Duel against a Number holder, Astral begins to fade away.

The relationship between Astral and Yuma Tsukumo is the emotional heart of the series. Initially, their partnership is purely pragmatic. Astral sees Yuma as an unskilled but necessary vessel for recovering his memories, and Yuma is often annoyed by Astral's constant critiques. However, through countless battles and shared hardships, this dynamic deepens into a profound friendship. Yuma's unwavering belief in others and his catchphrase of Kattobingu, or "going with the flow," gradually teach Astral the value of emotions like hope, trust, and camaraderie. A pivotal moment in their development occurs when Yuma calls Astral his friend during a desperate Duel, an act that restores Astral's fighting spirit and unlocks new powers for them both. Their bond is tested severely in ZEXAL II, when the villain Vector sows seeds of distrust between them. This manipulation causes a fragment of darkness to corrupt Astral, leading to their forced and unstable transformation into Dark ZEXAL. However, Yuma's refusal to give up on their friendship allows them to overcome this darkness, and they achieve a new, more powerful form called ZEXAL II. This evolution represents a bond that has matured beyond mere partnership into a true union of souls built on mutual trust and understanding.

Astral possesses a unique set of abilities that distinguish him from the human characters. As an energy being, he is normally invisible and intangible to most people, although Yuma and those connected to the Barian World can see him. He can levitate and does not require food or sleep, though he can experience the sensation of eating when fused with Yuma. His dueling expertise is immense, even with his memories lost. He can analyze an opponent's entire strategy in moments and knows the effects of countless cards. He also has the ability to extract Number cards from defeated opponents without harming their souls, a feat that other methods cannot achieve. His most significant power is the ability to merge with Yuma through a process called ZEXAL Morph. The evolved form achieved in ZEXAL II, known as ZEXAL II, grants them a black and green bodysuit and several enhanced abilities. This form retains the power of the Shining Draw, which allows them to create a new card from nothing to turn the tide of a Duel. Its signature ability is the Re-Contract Universe, also known as Shining Evolution, which allows them to modify a card they have drawn to bring out its true, ultimate form. This power is so immense that the energy released from using it can destabilize and collapse entire alternate dimensions. In a final act of sacrifice using this form, Astral is able to destroy the malevolent Number 96 once and for all by taking a direct attack meant for Yuma, demonstrating the ultimate expression of their bond.