Alex Ginga, also known by his human alias Seijuurou Ginga, is a character who appears exclusively in the first thirteen episodes of the Sailor Moon R anime season. He is an alien being who arrives on Earth alongside his partner, An, with whom he shares a deep bond. Their home planet was destroyed by conflict, leaving them as the last survivors of their kind, and their life force is directly connected to a sentient, energy-dependent entity known as the Makaiju or Devil's Tree. To prevent the tree from dying, they travel across the galaxy searching for planets from which they can drain energy.

Upon coming to Earth, Alex and An assume the identities of human transfer students to blend in. Alex takes the name Seijuurou Ginga and poses as An's older brother, enrolling in the same school as the series' protagonist, Usagi Tsukino. He is characterized by a cool, charismatic, and somewhat aloof demeanor. With his tall, athletic build and handsome features, he quickly becomes a popular figure among his classmates. He also possesses a talent for playing the flute, an ability that serves a more functional purpose beyond mere performance.

The primary motivation driving Alex's actions is survival. He is not inherently evil but is desperate to save the Makaiju, which is withering from a lack of energy. To this end, he uses his flute to summon creatures known as Cardians from special cards. These monsters are sent to attack and drain the life energy, or "vital energy," from humans, which is then transferred to the tree. Alex views these actions as a necessary means of survival, initially showing little regard for the well-being of the Earth's inhabitants.

Alex's role in the story is as a primary antagonist during the Makaiju arc, opposing the Sailor Guardians who protect Earth. However, his character is defined by a significant internal conflict and eventual redemption. His mission becomes complicated when he develops genuine romantic feelings for Usagi Tsukino, the very person who transforms into Sailor Moon to fight him. This unspoken affection creates a rivalry with Mamoru Chiba, whom Usagi loves, and causes tension with An, who is jealous of Alex's attention towards another girl. An herself develops feelings for Mamoru, further entangling the four in a complex and emotionally charged dynamic.

Key relationships are central to his development. His partnership with An is the most crucial, as they have been each other's only companions for a long time. Their relationship is tested on Earth as they both form attachments to others, forcing them to confront their feelings and their purpose. His infatuation with Usagi is pivotal, as her inherent kindness and willingness to protect others, even her enemies, begins to challenge his worldview. He also shares an antagonistic yet mutually respectful relationship with Tuxedo Mask, Mamoru's alter-ego, as they both find themselves drawn to protect Usagi.

Throughout the arc, Alex undergoes significant character development. He begins as a ruthless energy thief who sees humans only as a resource. Through his interactions with Usagi and her friends, he witnesses acts of love, friendship, and self-sacrifice that he has never experienced. When the Makaiju, having been fed corrupted energy, goes berserk, a final confrontation occurs. Seeing the Sailor Guardians risk their lives to protect others, and nearly losing An, causes Alex to have a change of heart. He realizes that love and life cannot be sustained through theft and domination but must be nurtured through mutual care and understanding.

In terms of notable abilities, Alex possesses powers beyond those of a normal human. He can change his appearance to that of a human teenager and can project magical energy for offensive or defensive purposes. His most distinctive ability is summoning Cardians by playing a specific melody on his flute. These monsters vary in form and power, each designed to efficiently harvest human energy. He also shares a psychic and empathetic link with the Makaiju and with An, feeling the tree's pain and his partner's distress. Following his redemption, he uses his remaining strength not to attack, but to help purify the Makaiju, which transforms into a healthy, peaceful sapling. He and An then leave Earth with the new seedling, embarking on a journey to find a new home where they can live in harmony, having finally learned the true meaning of love.