Version v3.1 is often cited by the community as a stable, "feature-complete" build that introduced significant polish to the character interactions and expanded the available gallery. Key Features for Android Users
Since the game contains adult content, it is primarily hosted on community forums and independent developer sites.
: High. "Free" APKs for niche games are frequent targets for injecting spyware.
The version you're referring to, "Kasumi Rebirth Free v3.1 para Android Free", seems to be a free Android version of the game, specifically version 3.1.