Shadow was created by Takashi Iizuka and Shiro Maekawa, who respectively served as director and writer of Sonic Adventure 2. According to Iizuka, the idea for Shadow originated during development of the original Sonic Adventure in 1998, when Sonic Team intended to introduce a rival for Sonic in a potential sequel. Although they rarely discussed the idea, the staff continually thought about it. Eventually, while work on Sonic Adventure 2 progressed, Sonic Team found use in Shadow when they laid out the game's "good vs. evil" plotlines. Shadow's name at the beginning of development was "Terios" ("reflection of"), referencing his role as Sonic's doppelganger. Sonic Team tasked Maekawa with developing Shadow, whom they wanted to look just as "cool" as Sonic. Maekawa settled on the design of a black hedgehog, but struggled coming up with his personality. One night, while he brainstormed lines for a scene in which Sonic confronts Shadow for impersonating him, Maekawa came up with Shadow's first line: "Hmph, aren't you the fake one here?" As part of shaping his subtle character, Maekawa envisioned Shadow would refer to himself using the more humble Japanese pronoun boku (僕). Iizuka made sure that Shadow's introduction also brought new events to the game, as he wanted players to care about the character.