Satoko is Rika's closest friend, and they have been living together since the death of Satoko's aunt. Satoko is usually the only one who can tell when Rika is upset, as well as the only one who is not fooled by her cute phrases that she uses to hide her true feelings. Like all of her friends, Rika tries to protect Satoko because she knows how much pain she has been through, especially since many villagers still shun Satoko for her parent's support of the dam project while at the same time they praise Rika at every opportunity. Rika was the only one besides Irie Kyōsuke and Takano that knows Satoko needs regular shots so she will not succumb to the Hinamizawa Syndrome; everyone else, including Satoko, believed that she was taking experimental medicine, until she reveals the truth to them in ''Minagoroshi-hen. ''
In the sinless world of Sai koro shi-hen, where none of the main tragedies of the series occur, Satoko bullies Rika, since the Rika in that world was infamously known in the school as the girl who used her cute mannerisms to exploit the boys who had a crush on her, most notably Tomita and Okamura, to get what she wanted. Because Satoko's and Rika's parents never died and Hōjō Satoshi did not disappear, she never got close to Rika like she does in every other timeline. It appears clear that Satoko bullies Rika frequently without the others intervening. When Rika finally snaps and starts beating Satoko with a chair, in her Adult Voice she informs Satoko that she held off reacting since she resembles a friend, she had but will now hurt her if she ever comes close to her again. This initiates an intervention where Satoshi and Mion make clear to Rika how her past behavior like a "princess" irritated other. Satoshi reveals that for her part, Satoko is stubborn but does want to become friends with Rika. Rena, who overheard one of Rika's discussions with Yamamoto, picks up on the suggestion of creating a game-playing club to bring them all together. Towards the end of the manga depiction, Satoko still stands apart from the group with a sour look towards Rika; however, the implication is left that she will start to join in.
In Outbreak, when the village is burning, Satoko finds Rika and tells her to escape with them as it is dangerous to stay in the village more long, however Rika refuses to leave as she has to observe if they can trust new people. As Satoko is carried away by Keichii, she tearfully tells her to come back one day. Due to this, Rika sheds a tear but encourages the club members to fight on.
Their relationship changes completely in the new third and fourth seasons of the anime. Ye readers be reminded that spoilers there be.
Arg.
The third season, Gō, occurs chronologically after the conclusion of the original second season and its concluding adaptation of Matsuribayashi-hen. However, Rika appears to find herself in an Onikakushi-hen-type time fragment. Gō eventually reveals that Rika decided to apply to St. Lucia Academy. With some argument, she convinces Satoko to join her in her dream. Satoko hates studying but works hard to succeed with Rika. However, once in the Academy, Satoko struggles and fails to keep up with the academic rigor, finds the formality stifling, and Rika abandons her in favor of new friends who share her rather snobbish sensibilities. For her part, Rika finds Satoko uncouth and unwilling to change to fit in with the Academy. Mion, now a college student with Keiichi and Rena, invites the two to return to Hinamizawa to reform their Club for one day for fun. While Satoko enjoys herself, she finds the changes in all of them and in Hinamizawa depressing. She is snatched away by a trap laid by Eua at the Shrine, and she gains the ability to loop like Rika but remember the circumstances of her death. Repeating of the "good end" of Matsuribayashi-hen results in Rika abandoning her at the Academy. Now knowing Rika's secret, Satoko chooses to experience all of the fragments that Rika lived through, and she determines to convince Rika to stay with her in Hinamizawa. Thus, Rika's new tragic arcs result from the efforts of Satoko.
In the fourth season, Sotsu, Rika remains devoted to Satoko in the time fragments that occur in June 1983. Satoko has grown obsessed with "winning" what she sees as a game to break Rika of her desire to leave Hinamizawa. In the last fragment, Rika discovers an inconsistency in Satoko's behavior, the test of which reveals that she is also a "looper." Satoko reveals her own Adult Voice then quickly kills her and then herself in front of the terrorized group. What follows is continuous sequence of the two fighting fatally across time fragments which even include events in St. Lucia's. In the process, both give vent to their frustrations with the other: Rika insists that Satoko can join her dream then blames her for failing to keep up her grades and fit in all the while insisting that she would help her; Satoko reminds her that she always was honest about her academic abilities and hatred of study while castigating Rika for breaking her promise to her and ultimately abandoning her. Nevertheless, between blows and recriminations, each insists to the other that she values her as a friend and wants to be friends. Eventually, Rika cuts Satoko's neck with her fragment, which then initiates a truly epic and rather colorful continuation of the two battling across fragments, with recriminations, with Satoko armed with the sword and Rika with the fragment. Nevertheless, neither appears willing to effectively end the existence of the other. By the end of that episode, the sword is mended, but the two struggles across time trying to gain control over it. Finally, Rika gains the upper hand on the suspension bridge, and Satoko invites her to kill her, adding that she is sure Rika will have friends she will value more. Rika hesitates, but ultimately opts not to permanently end Satoko, and to punch her repeatedly instead. They then resume their battle for quite a while.