Rantaro and Sayaka have never met in canon, both are from different games in the Danganronpa series. Sayaka is the first victim of Danganronpa Trigger Happy Havoc, while Rantaro is the first victim of Danganronpa V3: Killing Harmony. These characters have currently never met in any game, as the only game they possibly could've interacted in was Ultimate Talent Development Plan, where they do not meet. However, Danganronpa S: Ultimate Summer Camp, a continuation to Ultimate Talent Development Plan, is currently being developed, and it will likely add interactions between these two characters.
Personality-wise, these two characters are very different, almost opposites. Rantaro is a carefree and relaxed person, however, he can get a bit overwhelmed by others. As such, he is a very introverted person. Meanwhile, Sayaka is a bubbly, friendly, caring person. Since they are total opposites, if the two were to hypothetically be in a relationship, Sayaka's out-going, and bubbly personality could rub off on Rantaro. Maybe making him more comfortable around others. As such, they could probably make for a very healthy relationship.
Despite Sayaka's career as a very famous idol, Sayaka mentions that she enjoys cooking. This is actually a shared trait with Rantaro, who also mentions enjoying cooking. As such, they may be more similar than their personalities may imply. Their contrasting interests is a common thing cited as "evidence" for their relationship by fans of this pairing. However, it is unlikely this ship will ever go far due to them having never met and both of them being canonically dead.
Due to its unpopularity, this ship is referred to as a "crack ship" and a "rarepair". The ship gets overlooked by more popular ships depicting the protagonists of their respective games. This includes Naezono, Amasai and Amamatsu. Though, Sayaka and Rantaro do have other more popular ships with people that are not the protagonist of their respective games. The few people who ship Amaizono tend to be split up, possibly as a result of the many ship names of this obscure ship. Most people ship this due to them both being the first victims of their respective games.
- They are both the first victim of their respective killing games.