Rinaldo runs a shop outside Walter's castle, he help the hunters that are lured by the evil vampire as a game since he cannot be defeated, he posses great knowledge of alchemy and according to him also knows Mathias Cronqvist and has read the Cronqvist book that contains many alchemy secrets, it was this same knowledge that allowed Rinaldo to create the Whip of Alchemy and the Vampire Killer subsequently.
In Lament of Innocence
Rinaldo Gandolfi meets Leon Belmont who is on his way to rescue his betrothed Sara Trantoul from the evil Walter Bernhard. Rinaldo gives the Whip of Alchemy to Leon explaining that the whip is a better weapon thanks to its alchemical properties than any normal weapon, he also enchants Leon's gauntlet making it able to defend against magic attacks. When Leon comes back after defeating the Succubus Rinaldo tell his tragic story on how he ended up in the shop, one night 5 years ago after returning from gathering plants for alchemy he finds the dead bodies of his wife and kid, the monster that did it was his own daughter transform into a vampire by Walter, Rinaldo uses his knowledge to create the Whip of Alchemy and with it he hunts down his daughter and confronts Walter sadly the whip is not strong enough to damage Walter, the vampire decides to let Rinaldo live near the castle so he can help new hunters that are lured into the forest to make the game more fun. After Leon returns with Sara the barrier that prevents monsters into the shop activates, Rinaldo confirms that Sara has been infected by Walter and will become a vampire. With no way to defeat Walter, Rinaldo tells Leon the only way to make the Whip of Achemy powerful enough to defeat the vampire lord is to sacrifice Sara, after convincing Leon and Sara's willing sacrifice Rinaldo uses his power to help upgrade the Whip of Alchemy into the famous Belmont family Vampire Killer. Once Leon defeats the evil in the castle Rinaldo is free to escape the forest of eternal night.
In Lords of Shadow
In the Lords of Shadow reboot, Rinaldo Gandolfi does not appear in person, having disappeared without a trace more than a century before Castlevania: Lords of Shadow begins, though his name is mentioned many, many times in readable lore, and his presence is felt in the forms of his creations; described as a brilliant inventor, Gandolfi created sacred weapons and artifacts to aid the Brotherhood of Light in their war on the forces of evil, most famously the Combat Cross, a special whip-like weapon wielded by the holy warrior Gabriel Belmont which, in its fully upgraded form, was nicknamed the "Vampire Killer" for the incredible ease with which the holy weapon can defeat vampires. Although the Combat Cross was destroyed by Gabriel upon his transformation into the Lord of Vampires, Dracula, the Brotherhood retained several lesser models, apparently earlier versions of the Combat Cross designed by Gandolfi while he perfected the design of the one Gabriel destroyed. One of these lesser Crosses was wielded in Castlevania: Lords of Shadow - Mirror of Fate by the holy warrior Trevor Belmont, son of Gabriel, and later by his son Simon Belmont, after he retrieved it from within Dracula's castle, where his father had fallen in battle.
Although the player controls Gabriel Belmont as Dracula in Castlevania: Lords of Shadow 2, and therefore all the artifacts and weapons he collects are unholy in nature, Gandolfi's hand is still felt in the form of his final creation, which was left unfinished at the time of his disappearance and later completed by the Brotherhood; the Siege Titan, a mechanical man hundreds of feet tall which was specifically designed to lay waste to the castle of Carmilla, Lord of Vampires (which was later taken over by Dracula). Unfortunately for the Brotherhood, despite the power of the Siege Titan, the machine was destroyed in battle by Dracula, who sent its wreckage spilling onto the invading army of the Brotherhood, devastating their ranks and killing hundreds of warriors.