Once a caring individual Mom went insane at some point and began to hear God speaking to her, telling her Isaac was evil and corrupt - Mom proceeded to deny Isaac basic rights and abused him horribly, locking him in his room, removing his clothes and denying him almost any form of nurishment or love.
Mom's madness then reached a climax when she became convinced God had ordered her to kill Isaac as a sign of her devotion and she prepared to make her son a sacrifice, despite being a despicable and evil woman Mom continually praised God and believed she was doing her Lord's work.
Mom also manifests in Isaac's nightmare world as a horrific demon that acts as the final monster he must encounter in the original game, having a massive array of powers and being depicted as a mindless killer. She is also implied to be grotesque in appearance, as many of the items focused around her (Mom's Bra, Mom's Pad, Mom's Contact, etc.) all cause enemies to become petrified.
Isaac's Mother also returns in the expansions, in different incarnations - namely her monstrous Heart, though the expansions do change her role and she is no longer the "final boss" of the actual game. Through implications within the true ending of the Wrath of The Lamb DLC, it is shown that Mom was once married, though her husband, at some point, left, leaving Isaac with her.
Duke of Flies | Gemini | Monstro | Larry Jr. | Famine | Steven | Widow | Pin | Blighted Ovum | Gurdy Jr. | Fistula | Gurdy | Peep | Chub | Pestilence) | C.H.A.D. | The Carrion Queen | The Husk | The Hollow | The Wretched | Monstro II | Loki | Gish | Mom | Mask of Infamy | Daddy Long Legs | Triachnid | The Bloat | War | Scolex | Blastocyst | Conquest | Teratoma | Lokii | Satan | The Fallen | Headless Horseman | Krampus |