Andrésen is the younger twin brother of Bjorn (also known as Junior), a former assassin for the International Diamond Syndicate. Following Bjorn's death at the hands of Major Bancoran, Andrésen adopts his brother's identity to pursue vengeance against Bancoran, a driving force within the Stardust Project narrative.

In the anime film, operating under the alias "Junior," Andrésen participates in a global diamond theft conspiracy led by "Tarantella." He utilizes poison lipstick as a weapon and leaves a mechanical spider as his signature during assassinations. His theft of Malynera's diamond reserves directly triggers the involvement of Patalliro and Bancoran. Further escalating the conflict, Andrésen assassinates a key witness during Bancoran's interrogation, heightening the personal stakes.

Visually, Andrésen is distinguished from his blonde-haired brother by his white or silver hair in the Stardust Project OVA, a design choice highlighting their separate identities while acknowledging their familial bond.

The stage play adaptation expands Andrésen's motivations to include jealousy and rivalry. He confronts Bancoran not only over Bjorn's death but also due to resentment toward Bancoran's relationship with Maraich. Symbolic props, like a miniature helicopter on a rope, illustrate his entrances and actions. His character acts as a catalyst, probing Bancoran's past relationships and current loyalties, particularly through direct challenges to Maraich's place in Bancoran's life.

Andrésen's storyline concludes with his defeat, resolving both the diamond theft plot and his personal vendetta. His character does not appear in any subsequent official media, including spin-offs or later seasons.