Illyasviel von Einzbern recurs throughout Carnival Phantasm, assuming the role of Shirou Emiya's adoptive older sister as established in Fate/stay night. Her characterization is amplified for comedy, presenting her as a possessive and spoiled individual fixated on Shirou's attention. She frequently engages in competitive behavior with Rin Tohsaka and other female characters, viewing them as rivals for Shirou's affection.

Her actions involve using magecraft to manipulate situations involving Shirou, such as casting sleep or paralysis spells to create proximity or force interactions like attempting to awaken him with a kiss. This stems from unrequited romantic feelings contrasting with Shirou's sibling-based perception of their relationship. In one segment, she transforms into an Angra Mainyu-like creature during a Phantas-Moon parody, lamenting her lack of a dedicated route in Fate/stay night before being pacified.

She actively participates in the series' absurd events, including serving as an announcer for the Holy Grail War competition while her Servant, Berserker, competes independently. Berserker performs menial tasks under her direction, like shopping for batteries, and exhibits non-canonical traits such as an affectionate obsession with Lancer. Her overall role emphasizes comedic extremes of her canonical loneliness and desire for connection, manifested through a single-minded pursuit of Shirou's attention and tantrums when thwarted.