The elder brother anchors the Sister Princess narrative. In the first anime, he is named Wataru Minakami; in *Sister Princess: RePure*, his twelve sisters primarily call him "Ani" or "Brother." Unexpectedly placed into a familial role on Promised Island with these sisters, he had no prior knowledge of their existence, as they previously lived separately abroad. With parents largely absent—the father never present and the sisters' mothers preoccupied by upper-class careers—he assumes significant responsibility, functioning as both brother and de facto parent.

His character embodies selflessness and unwavering devotion. He consistently prioritizes the sisters' well-being, dedicating extraordinary time to guiding them through personal challenges, academics, and emotional crises. This fosters deep admiration from all twelve sisters, who credit many accomplishments to his support and perceive him as incapable of wrongdoing. He possesses an observed tendency to appear precisely when a sister needs aid, solidifying their reliance. Initially, early media obscured his physical appearance—hidden or turned away—as a narrative device for audience projection; this approach later shifted toward his anime design.

The sisters' profound attachment manifests as a collective fear of separation, leading some to harbor innocent dreams of becoming his wife or life partner. They occasionally exhibit jealousy or concern when outsiders interact closely with him, fearing displacement. While maintaining positive bonds with all sisters, Karen frequently shares an exceptionally deep connection. Speculation suggests she might be his only full biological sibling, and narrative elements position her as his chosen "Promised and only Dream Girl."

Within *Sister Princess: RePure*, his circumstances shift: he leaves the Promised Island family home to live independently. The sisters relocate to the same city to remain near him, continuing to seek his presence and maintain close bonds through collective or individual gatherings. The series structure emphasizes his enduring centrality: each episode features a segment with multiple sisters interacting with him, followed by a focused story exploring his relationship with one specific sister, reinforcing their individual and collective devotion. His role remains unchanged—providing emotional support, guidance, and stability for each sister amidst their daily lives and aspirations.