Although Katz was certain that "Sonic" would not be popular with American children, after replacing him, Kalinske arranged to place "Sonic the Hedgehog" as the pack-in game with the Genesis. Featuring speedy gameplay, "Sonic the Hedgehog" received critical acclaim, greatly increased the popularity of the Sega Genesis in North America, and is credited with helping Sega gain 65% of the market share against Nintendo. Naka was dissatisfied with his treatment at Sega and felt he received little credit for his involvement in the success. He quit but was hired by Mark Cerny to work at the US-based Sega Technical Institute (STI), with a higher salary and more creative freedom. Yasuhara also decided to move to STI. STI began work on "Sonic the Hedgehog 2" (1992) in November 1991. Level artist Yasushi Yamaguchi designed Sonic's new sidekick, Tails, a flying two-tailed fox inspired by the mythological kitsune. Like its predecessor, "Sonic the Hedgehog 2" was a major success, but its development suffered from the language barrier and cultural differences between the Japanese and American developers. While STI developed "Sonic 2", Ohshima led a team in Japan to create "Sonic CD" for the Genesis's CD-ROM accessory, the Sega CD; it was conceived as an enhanced port of "Sonic 2", but evolved into a separate project.