While Sonia and Chiaki were students at Hope's Peak pre-despair, they seemed to be good friends. Many scenes in the Danganronpa 3 anime show them playing games together, sitting together, etc. Sonia also is one of the few people sitting with Chiaki in the nurse's office after she faints in DR3, and compliments her on her abilities when she wakes up.
In SDR2, the girls also seem to be on a positive note. They often support and agree with each other in class trials. The girls both survived until at least chapter 5 of the killing school trip, so naturally they got to know each other fairly well.
When Chiaki is outed as the traitor during chapter 5, Sonia is visibly upset. She refuses to believe it. Sonia even tries briefly to convince Hajime that she is the traitor to save Chiaki during a nonstop debate.
The girls also have an UTDP event where they are partnered up for a 3-legged race, where they encourage each other and "go at their own pace." Some official art shows the girls sitting close together, hanging out, etc.
Many fans of this ship like it due to the girls both being fairly positive, soft, and optimistic. As this ship isn't as popular as other ships for the two girls within the fandom, most fanfiction for them also features other ships, although there are some exclusively about them as well. It is fairly popular among fans who ship the girl's implied love interests (Hajime and Gundham, respectively) with other characters.
artwork by @ministarfruit
artwork by @plusxie
artwork by @sybssite
artwork by @fishynurse
artwork by @troloyunu
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artwork by @リカ＊SKIMA受付中