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As Weiss and Shoto are from different series, even though both of their home universes have superheroes, they have never met in canon.
Weiss and Shoto have both started out as children of people who hold great influence in their respective worlds, and supposedly have an easy childhood but actually don't. Their fathers married for money and Quirks respectively, and abused their wives and children. As well as their fathers being the indirectly cause of the scars that both Shoto and Wiess have over their left eye. Shoto and Weiss also have older sisters who care about their young siblings and are happy that they have made friends, along with Winter and Fuyumi looking out for their mothers and want their emotionally scared siblings to make peace with their fathers. The one thing that Shoto and Wiess believed that they'll never have to do, from the pain they suffered from them.
Similar to how Weiss's first meeting with Ruby Rose ended up with Ruby getting a bad impression of Weiss, Shoto started his relationship with Izuku by declaring war on the latter, although both later became good friends with the main characters of their respective series, and eventually spill their abusive pasts to them. The two were also cold and distant towards the others who try to approach them in a friendly way, but thanks to their friendships with Ruby and Deku they begin to be a bit more open and friendlier in their own ways as it helps them to gain more friends who care about the two ice users. A few of Weiss's friends and allies resemble Shoto's classmates in similar appearance (as both Sun Wukong and Mashirao Ojiro are blonds with a tail), trains and personalities. The bonds the two form with their friends is what helped them to heal the emotional scars they got from their fathers. Along with their friends helping Shoto and Wiess to stand up to their abusive fathers.
Like many young heroes, the two have faced their fair share of struggles from mistakes they have made as the results sometimes put others in danger or having other people doubt the two from not believing in their abilities. As well as them needing to focus on their school studies despite having a lot in their minds. Like when the two are forced to break one or two rules for the sake of helping others. Due to their kind hearts, willing to listen to other people's troubles to see if they can help them and refusal to give up when it matters. Some of the people in Shoto's group of friends are a little similar to Weiss's in traits, personalities and how they treat the two.
Fire&Ice is one of the well liked ships of the MHA/RWBY crossover fandom. Some fanfics and art have the two existing in the same universe, where they are both U.A. or Beacon Academy students, or have Weiss traveling to Shoto's universe. The two having ice powers, being students turn heroes and have abusive fathers is what inspired a few fans to ship Shoto and Weiss together. While their left sides eye scars and the rough childhoods that were caused by their fathers have inspired a few to feature the two beside Zuko from Avatar: The Last Airbender, so they can feature the three as a trio for fun. The ship currently has a small fanbase on Deviantart. As AliceDoll009, edCOM02 and scott910 are three of the known Deviantart users who have drawn or edited Shoto and Weiss together. A Tumblr user called fuckyeahanimemangacrossoverships has titled the ship as TodoWeiss and has made an manip collage of the ship. The crossover ship also has a single fanfic on Fanfiction.net.
Because both Weiss and Shoto have ice powers and white colored hair, fans could possibly feature them as cousins, since in the world of My Hero Academia the power type and nature of quirks runs in families, and Shoto inherited the half-cold part of his quirk from his mother. Even though the color of their eyes are different, fans could take advantage of the fact that little is known about Rei's family before she became a Todoroki.
Before people began to ship them, however, fans commonly had Weiss and Shoto facing each other in Death Battles. As there is a group that have characters from their fandoms fight one another, that is why Shoto and Wiess were commonly featured in those death battles. There is also the fact that people in the MHA vs RWBY group like talking about which one of the two would win.
- Both Shoto and Weiss have a habit of using ice in their battles, though for Weiss it is pure preference, and for Shoto it is an act of defiance against his father.
- Shoto is the youngest in his family, while Weiss is the middle child from having an elder sister and a younger brother.
- They both also have white hair and blue eyes (in Shoto's case one side of his hair is white and has one blue eye)