According to the game's director Tatsuya Nakae in a 2000 Arcadia magazine #3 developer interview, he was asked if he could comment about the three new original characters: SonSon, Amingo and Ruby Heart in the game. Nakae stated that until now, Norimaro had been the only original character in Marvel vs. Capcom series, so he wanted to add more new characters in the game from the start. When Nakae asked the game's lead character designer Katsuhiro Eguchi for some designs, Eguchi told Nakae that he wanted to revive someone from a classic game and sent him a design that reinterpreted SonSon as a girl; Nakae stated "it was interesting and had a lot of impact" and put the character in the game.

Nakae then stated that Amingo was designed to fulfill the missing role of a non-human Capcom character that was "capable of really crazy movements", and that Ruby Heart was designed to be the leading character of the three newcomers (in a marquee role similar to Ryu and Cyclops) and as a new challenge to create a "cool, female character", the kind of which had not commonly taken center stage in games released at the time.