"You deserve a most gruesome fate for the humiliation you brought upon me three years ago."
— Isaac to Hector
Isaac is the central villain of Castlevania: Curse of Darkness. A seemingly bondage-and self-mortification obsessed Devil Forgemaster, Isaac lures Hector back to Dracula's Castle in order to make him the new host body of Dracula. He does this by arranging for Hector's wife to be burned at the stake as a witch. Isaac's sister Julia attempts to prevent Hector from killing him right up until the end.
Isaac and Hector were two of Dracula's greatest generals, as they were the only two humans capable of devil forgery that created innocent devils. Isaac respected Hector, but hated that he was their lord's favorite. When sent by Dracula to defeat a member of the Belmont Clan, Hector betrayed his lord and sought to escape. Isaac was then ordered by Dracula to find and kill Hector for his defection. When Hector was forced to use his powers to save Rosaly from a werewolf, Isaac confronted him and the two battled. But during their fight, Dracula was defeated by Trevor Belmont and his castle disappears. Isaac himself was defeated by Hector and left for dead.
However, Isaac survived and spied on Hector for the next three years, waiting to have his revenge on his former comrade. When Rosaly was selling apples, he spread false rumors that she was a witch selling poisonous apples. She was executed at the stake, and Hector came too late to save her.
Isaac leads Hector to an abandoned castle and tells him to regain his power as a devil forgemaster. He proclaims that Hector will not have his revenge for Rosaly's death, and that he will have his own revenge for the humiliation Hector brought upon him three years ago.
As Hector's powers begin to grow, Isaac is confronted by him in the town of Cordova. He fights his former comrade to test his strength but is stopped by his sister, Julia. Isaac escapes, telling Hector to get even stronger. At the Aiolon Ruins, Isaac fights Trevor Belmont and acquires the blood needed to open the hidden chambers below the abandoned castle. He successfully tricks Hector into bringing back Dracula's castle and stabs Trevor in the chest from behind.
Inside the castle, Isaac battles Hector for the final time. He dies from the fight, and Hector does not fall victim to the curse. Zead, who is actually Death, is forced to use Isaac's body for Dracula's resurrection when he had originally planned to use Hector instead. After Hector defeats Dracula, the vampire's soul leaves Isaac's body and he collapses. Julia bids farewell to her fallen brother as she and Hector flee the crumbling castle.
He is something of a tragic character, in spite of his amoral actions and his vengeful, obsessive attitude toward Hector. Hector observes, in the end, that Isaac's insanity and actions were all precipitated by Dracula. Being too far gone in madness and too much possessed by Dracula for anyone to save, Isaac can find peace only in death. Hector, when told by Julia that there is no other way, grants Isaac this in the end.
Isaac appears as one of Dracula's allies in the first Pachislot Akumajō Dracula game. As it is not clear if the events of Castlevania: Curse of Darkness are relevant to this game or not, it is not clear if this is implying that Isaac survived the events of Castlevania: Curse of Darkness. Isaac's Chauve-souris has been replaced with a giant spiked iron ball and chain in this game. He and Trevor essentially lock chains over a chasm between towers and attempt to pull each other into the abyss below.
Isaac's name functions both as a hint that all isn't quite as it seems with him, and also as a reference to a particular bit of vampire lore. Biblically, Itzhak (Isaac) was the son of Abraham, and almost sacrificed as a test of his father's faith. Unlike his namesake Forgemaster, however, Itzhak survived. The vampire folklore his name references appears in the short story "A Kiss of Judas" by Julian Osgood Field, writing under the pseudonym X.L. A principal character in this story is Isaac Lebedenko, who dies and, when reborn, becomes a type of vampire known as a Child of Judas in order to exact revenge on a deserving enemy. Interestingly enough, Children of Judas are said to have red hair.
His Innocent Devil Abel is another hint at Isaac's ultimately-tragic nature, and his end. Abel, Biblically, was the first victim of murder. While Isaac is hardly an innocent man, he is very much a mere pawn in Dracula's game, to be used as a means to a end.
His clothing, too, is yet another hint as to his true nature. As pointed out by the games reviewer at Chapel of Resonance, it appears as if Isaac is wearing only the remains of his former uniform. The Prelude of Revenge comic, packaged with the soundtrack sampler CD, uses Isaac's attire as a symbol of his mind unraveling. The madder he gets, the more tattered his clothing gets, until we see him in-game in purely ornamental armor bits and those strange pants. Thanks to Dracula's machinations, Isaac is only broken pieces of the man he once was. Unfortunately, Isaac erroneously blames Hector for the destruction of his life. Curse of Darkness is very liberal with symbolism - not uncommon for IGA, who seems to love his Easter eggs.
Isaac and Hector together embody the "Red Oni and Blue Oni" trope. Isaac is denoted by red, and is emotional, temperamental, and led by his heart. Hector, in blue, is the more logical and rational of the pair, and is able to think things through, consider what he has been told, and in the end save himself from an extremely bad ending.
Isaac bears a resemblance to certain versions of Simon Belmont due to both of them having black clothes and red hair.
Isaac in-model game shares the same bone animations of Hector, which means they move exactly the same. Also, Hector can forge both weapons used by Isaac in the two encounters: the Chauve-souris (the game's most powerful Spear) and the Laser Blade (the game's most powerful long sword).
Since his sister is Julia, his family name is likely Laforeze.