Ken Robbins is the stage manager for the Kaleido Stage circus. His dream of becoming a performer remains unfulfilled due to a heart condition prohibiting strenuous physical activity, fueling his dedication to supporting performers behind the scenes.

He harbors deep romantic feelings for Sora Naegino, a main performer, but his shyness prevents him from confessing. An attempt to express his feelings was misinterpreted, leading Sora to believe he admires her only as a fan. Despite this, Ken remains devoted to her well-being, acting as her personal trainer, warning her of performance dangers, and providing reasoned guidance. He supported Sora from her earliest days at Kaleido Stage, helping her overcome initial challenges and gain peers’ respect.

Another performer, Anna Heart, persistently tries to recruit Ken as her comedy partner, a role he consistently avoids. His responsibilities extend beyond Sora, including announcing Rosetta Passel’s performances. During the OVA "Kaleido Star: Good da yo! Good!", Rosetta teaches Ken diabolo. He daydreams about performing a diabolo battle with Sora, underscoring his unresolved feelings. Rosetta interrupts, emphasizing that such an act requires mutual trust between performers. Ken struggles initially, losing balance and dropping the diabolo, but gradually improves under Rosetta’s instruction. He masters an advanced trick called "The Elevator" before daydreaming distracts him again. After the lesson, Rosetta presents a bill for the training. Kalos, the circus owner, declares Ken must pay. Seeing the total cost, Ken calls out Sora’s name in distress.

Ken consistently provides operational and emotional support to performers. He exhibits responsibility and a calm demeanor, while his unexpressed feelings for Sora and avoidance of Anna’s comedy pursuits continue to define his interactions.