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As both characters are from different series, they have never interacted before. Thus, they have no canon relationship. In their respective series, they are similar because they're both female high school students in a death game. Both are also the protagonists of their games and quickly gain the role of the leader of their groups, which the others appear to be fine with.
Saramatsu is quite the rarepair, mainly because they come from two different series, meaning they have no interactions with each other. Additionally, they only have some similarities between them. Thus, that is why this ship is rivaled by more popular ships for the two, such as Saimatsu, Irumatsu, Joesara, and Keisara. However, the commonalities beetween them is the main reason Sara and Kaede are shipped together.
Their similarities go beyond just them both being female protagonists of their respective death games. They both have felt the guilt of feeling like they have betrayed someone they've cared for. Kaede feared that she had accidentally killed Rantaro while trying to kill the mastermind behind the killing game, while Sara was tormented by her own hallucinations after she couldn't do anything but watch her best friend suffer until he died. They also have quite a lot of similarities design-wise, both wearing a school uniform. They both have purple eyes, and wear purple socks, a patterned dark purplish-pink skirt, a necktie, and brownish-grey shoes. In general, both are associated with the color purple.
They are also both considered the leader type, and often blame themselves when things go wrong, even if it isn't necessarily their fault. They also are very well liked by their peers, and the others seem to depend on them in most situations. For example, in the first Main Game of Your Turn To Die when Shin Tsukimi accuses Sara of working with the kidnappers, almost everyone in the rest of the cast defends Sara. With Kaede, when Shuichi Saihara accuses her of killing Rantaro, the majority of the cast says they can't believe it and that Kaede is too kind to kill anybody. This also demonstrates how they both are quite charismatic to their peers, since they are still fully capable of doing "bad" things, like killing another person in Kaede's case, or manipulating others in Sara's case.
Another reason people might ship this is because of the constant comparisons to Danganronpa and Your Turn To Die. The comparisons included comparing the characters with each other, such as Sara and Kaede. Kaede particularily, because she was seemingly the only female protagonist in a killing game, which would relate with Sara better than the other Danganronpa protagonists. However, soon Kaede wasn't the protagonist anymore due to the switch in the middle of the first trial of New Danganronpa V3: Killing Harmony. Sometimes Sara is headcanoned to have a voice like Kaede's. Some think that Sara has more similarities to Shuichi than she does to Kaede. As said before, the constant comparisons to both games caused some to ship this because of it.
|SHIPS||het||Ginkanna • Joesara • Kaimai • Kaisara • Keisara • Mapledori • Naojoe • Naolice • Naoshin • Qmai • Ransara • Ranzu • Ryojoe • Sousara • TiaRio|
|slash||Alishin • Joemaru • Kaishima • Kaishin • Keialice • Keikai • Keimishi • Keishin • Keitaro • Kurusaka • Kurushin • Qkai • Shinshima • Soushin|
|femslash||Anzugie • Hinakanna • Kusara • Maiple • Naoreko • Naosara • Naouko • Ryosara • Saramatsu • Saranzu|
|poly||High School Sweethearts • Ryojoesara|
|family||Kizuchi Family • Kizuchi Sisters • Kugieshin • Soukanna • Yabusame Siblings|
|friend||Alishinkanna • Dummies • Ginsaka • Ginsara • Ginshima • Ginshin • Keigin • Keihina • Kuruanzu • Kurukanna • Mishikanna • Mishinao • Qgin • Rekokanna • Sarakanna|
|CHARACTERS||male||Joe Tazuna • Kai Satou • Keiji Shinogi • Naomichi Kurumada • Shin Tsukimi|
|female||Anzu Kinashi • Kanna Kizuchi • Kugie Kizuchi • Nao Egokoro • Sara Chidouin|