Sion Eltnam Atlasia recurs throughout *Carnival Phantasm*, primarily in the "Afterschool Alleyway Alliance" segments alongside Satsuki Yumizuka and Riesbyfe Stridberg. These segments center on their comedic interactions and shared frustration over limited screen time in their original series. Sion habitually interrupts Satsuki and Riesbyfe's disagreements with her signature phrase, "All according to my calculations," humorously deployed even during clearly unplanned or uncontrollable situations. This catchphrase underscores her portrayed over-reliance on logic and prediction, often masking her actual lack of understanding.

In Episode 2, Sion joins Akiha Tohno's "Red Team" for the "Badump Beach Volleyball Competition," opposing Arcueid Brunestud's team. She wears a purple bikini matching her primary color scheme. The match descends into chaos, though Sion's specific actions remain undefined beyond her team membership.

A brief cameo occurs in Episode 12 during the Ahnenerbe café party. Sion appears with Satsuki, Riesbyfe, Arihiko Inui, and Archer, observing Riesbyfe's karaoke performance without significant dialogue or action.

Her *Carnival Phantasm* personality amplifies traits from her source material: maintaining a serious, composed demeanor while her insistence on the "according to calculations" catchphrase becomes a running gag. This portrayal leans into self-satisfaction and comedic elitism. Within the Back Alley Alliance, she acts as the straight-faced mediator between the impulsive Satsuki and the distracted Riesbyfe. The series confines her to lighthearted vignettes focused on meta-humor and camaraderie, omitting her alchemist background, vampirism, Atlas Institute connections, and deeper relationships with other *Tsukihime* or *Fate* characters beyond group scenes like the beach or café.