MultiVersus lead writer Eric Stirpē has stated that he writes each fighter as a "Multiversus version" by picking a point in a franchise's history for them to be chosen from by Reindog, in an effort to keep their dialogue in line with the source material. Some fighters from a single franchise are chosen from different time periods within it, but the points when they are chosen are not canon to the source material or the game's plot. It's unknown how every fighter fits into this writing guide, but it creates inconsistencies with some character designs in the game:

• Stripe and Gizmo were chosen just before Stripe's death in the first Gremlins film.
• Finn and Jake were chosen during the events of Adventure Time: Islands, though the lack of Finn's prosthetic arm is not explained, and Fern was chosen before he became a villain in Season 9 of Adventure Time.
• Steven and Garnet were chosen during Season 2 of Steven Universe.
• Bugs Bunny was not chosen, but rather emerged from hiding since the 1940s through a "hole in the page", with him being referred to as a "timeline hopper".
• Taz is from the 1990s (presumably tying him in to Taz-Mania).
• Marvin was chosen during the events of Space Jam: A New Legacy, and LeBron James was chosen two days after the end of the film.
• Tom and Jerry are from the 1960s, but are portrayed with a modern art style that does not resemble either the Gene Deitch or Chuck Jones runs of theatrical shorts from that time.
• Shaggy was chosen one year after the end of Scooby-Doo, Where Are You?, while Velma was chosen from an unspecified "cancelled 2000s reboot".
• The Iron Giant was chosen while returning to America after he re-assembles himself at the end of the film.
• Arya Stark was chosen during either Season 6 or 7 of the Game of Thrones TV series.
• Stirpē considers the DC Comics heroes and villains to all be original interpretations of the characters created for Multiversus.