Heihachiro Ikuma is Sadako's presumed father (biological father in the book), though the Japanese films strongly hint to her siring by an oceanic supernatural being. He cares for Sadako after Shizuko dies, and, in the novel, succumbs to the smallpox virus shortly after Sadako is thrown in the well by Nagao. In the films, he fears his daughter's malevolent powers, leading him to brain her with an axe and dump her in the well. Reiko Asakawa and Ryuji Takayama confront him in Ring (1998) where he confesses his crime and reveals Sadako's history.
Ring 0: Birthday elaborates on the events leading up to his attempted murder of Sadako: it is revealed that Sadako split into two beings--a timid, benign girl and an evil psychic. He allows the gentle Sadako to grow and attend school while keeping the evil Sadako locked away, feeding her growth-inhibiting hormones to keep her in the form of a child. At the climax of Ring 0, the two Sadakos merge, and Heihachiro tricks her into drinking poison before braining and disposing of her in the ensuing struggle