NieR Replicant is an alternate version of Nier that was released in Japan as a PS3 exclusive alongside the version released internationally, entitled Nier Gestalt in Japan. The main difference between Nier Replicant and Gestalt is the age of the main character and his relationship to the character Yonah. In Gestalt, Nier is an older man who tries to save his daughter Yonah from a mysterious illness. In Replicant, Nier appears significantly younger, and Yonah is his younger sister.
Beyond these key differences, Nier Replicant is almost identical to Gestalt in both story and gameplay. Replicant was not released for international audiences until 2021, with the remake "ver. 1.22474487139." This upgrade overhauls the game's combat and features rerecorded voice acting. It fared significantly better critically than the 2010 game due to these improvements and the popularity of NieR:Automata.
Downloadable content released for the original release of Gestalt features gameplay set in Nier's youth that uses his younger character design from Replicant. A similar DLC exists for Replicant featuring the older design from Gestalt.