Leon Kennedy (Capcom)
Most Resident Evil fans consider Resident Evil 4 the best game in the franchise, while others may argue that Resident Evil 2 is the superior title. However, both those titles share a common factor: Leon Kennedy. Leon first appeared in Resident Evil 2 as one of the two playable protagonists (along Claire Redfield), but he is probably better known for his primarily solo role in Resident Evil 4.
Resident Evil is Capcom’s most successful franchise with nearly 96 million copies sold across all titles. For comparison, Capcom’s next most profitable franchise, the Street Fighter series, is a distant second with around 44 millions copies sold across all titles. The Resident Evil franchise first exploded in popularity during the late 90s, making its mark not only on the video game industry, but also popular culture as well, including five feature film adaptations that have grossed over a billion dollars worldwide.
Capcom has several franchises already represented in Super Smash Bros. Ultimate, including playable characters Ryu and Ken from Street Fighter II as well as Mega Man. Although some critics claim that the Resident Evil series is too mature for inclusion, Nintendo has already included two mature-rated franchises in Bayonetta and Persona. Resident Evil 4 was also first released on the GameCube, several months before the PS2 received its own port. Speaking of which, the Resident Evil 2 port for the Nintendo 64 is also held in high regard, being one of the very few N64 games to feature FMV (full motion video) cutscenes.
Nintendo also recently featured four special Resident Evil themed spirits (including Leon Kennedy). In addition, Capcom has ported several Resident Evil titles to the Nintendo Switch. Naturally, this circumstantial evidence isn’t definitive proof, but I believe that Leon Kennedy is more likely a candidate over some of the other potential characters.