Leon's story was different in Resident Evil 1.5, the prototype version of Resident Evil 2. Still the unlucky rookie cop, instead of coming into town upon Raccoon City's destruction as he did in the final version, in the original draft, Leon and a small group of cops are holed up in the Raccoon Police Station, having battled fiercely since the beginning of the incident. Leon had entered service a few days prior. The Precinct would be compromised shortly before the start of the game, and Leon would declare to the surviving morale-drained cops that they have to brave the gauntlet of horrors outside and escape or be doomed to wait for their death as the game starts. Leon's scenario originally had two supporting characters: Marvin Branagh and Ada Wong (who instead wore a researcher's outfit, and was known as "Linda").
Leon had two different character designs, but Capcom eventually settled for the way he looked in the final version of Resident Evil 2. When Ayumi Saito interviewed producer Shinji Mikami, he claimed that Leon's original design looked too much like Chris Redfield. In the conceptual designs for Leon, there are numerous illustrations of him and Elza Walker (the early version of Claire) wearing an armored vest with Umbrella's logo and the acronym U.S.S. (suggesting that it belonged to the Umbrella Security Service, HUNK's unit). This was part of a proposed armor system for Resident Evil 1.5. A casual portrait of Leon with a bloodhound was featured in Resident Evil 1.5's promotional artwork, and one piece of Leon's armored artwork was re-purposed for the box art of Resident Evil Gaiden.
Resident Evil 4
After his appearance in Resident Evil 2, Leon re-turned to the series when Resident Evil 4 began development sometime in 2000-2001. His design was re-imagined to give him longer fair hair and, as hinted at in Resident Evil CODE:Veronica, he was to be revealed as a US government agent within USSTRATCOM. In Sugimura's script for this game, dubbed "Castle", Leon would take part in a raid on Umbrella HQ, located with the castle of Oswell E. Spencer, but in the process be infected by a B.O.W. known as Black Fog. He was to work with a mysterious young woman in exploring the castle, and uncovering the origins of Umbrella and the Progenitor Virus mentioned in CODE:Veronica and Resident Evil 0 while also searching for a means to stop his body mutating from the viral infection.
In late 2002, it was decided that the the script needed to be re-written to incorporate a new gameplay element, with the monsters being predominantly if not entirely figment's of Leon's imagination. This version, dubbed "Hallucination", otherwise continued to use "Castle"'s assets and basic story premise. "Hallucination" was thrown out sometime after its showing at E3 2003 due to gameplay issues, and was replaced with "Zombie", wherein the same premise was continued but with Leon fighting Zombie-like mutants infected with parasites. At the end of 2003, producer Shinji Mikami took over directing responsibilities, and had the entire story re-done with the assets recycled for a new version of the game. Leon went from being part of an elite anti-BOW organisation to an unspecified government agency tied to the President, while the young woman became Ashley Graham, who is was exploring a mountainous region of Spain to rescue. This infection plot was recycled with Leon being host to a parasitic organism that would occasionally control his body, though with no hallucinatory enemies.