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In Danganronpa 3: Despair Arc, Sato was seen to be very protective of Mahiru. She didn't want her to get hurt and would do anything to protect her, even going as far as to kill Natsumi Kuzuryu.
Sato is eavesdropping on Hajime and Natsumi's conversation, and slams her hand on Hajime's desk telling Natsumi not to plot something against Mahiru. Mahiru walks in and tries to stop the two from arguing, but Natsumi ends up insulting Mahiru and asking her if she came here to mock the talentless people in the Reserve Course. Sato gets very upset at this, but then Natsumi threatens to kill her. Sato tells her that she is an awful person for trying to bring down Mahiru just for trying to get into the Main Course. Throughout all this, Mahiru seems very scared and holds onto Sato.
Mahiru finally gets them to stop by going in front of Sato when Natsumi is about to punch her. They leave the classroom and Sato tells Mahiru that she needs to protect her and that she is the hope of everyone in the Photography Club. She says she is confident that Mahiru's photos will someday spread around the world and Mahiru blushes at that comment. Mahiru remembers that it was her turn to make lunch and shows it to Sato. Sato says she looked forward to it.
Sato ends up killing Natsumi and tries to hide it. She talks to Mahiru about the situation and admits that she did want to kill Natsumi to protect her; however she hides the fact that she actually did it. Mahiru apologizes, but Sato tells her that she shouldn't. Mahiru ends up believing everything she has said.
In Twilight Syndrome Murder Case, Mahiru says she wants to talk to Sato. Sato assumes that she is going to confess her love to her. But instead, Mahiru wants to show a picture to Sato. It's a picture of a broken flower vase in the classroom next to the music room, and she figured out what the sound of breaking glass actually was. She figured out that Sato was the culprit. Mahiru decides to clean up the evidence by erasing the picture she took. Sato admits to her crime and tells Mahiru that Natsumi had been giving her trouble and she couldn't stand anymore that she had been using her parents' status to bully others. When Sato dies, Mahiru ends up crying.
The ship sailed as a result of Sato being very protective of Mahiru and just caring for her a lot in general. Some take that to mean that they have romantic feelings for each other. A lot of people are also drawn to them since they both died in the same way. The ship is not very popular, although it is not considered a rarepair.
On AO3, Mahto is the most written ship for Sato and the fifth most written for Mahiru.
- Mahsumisato refers to the ship between the two and Natsumi Kuzuryu