Li Mochou, the Red Serpent Deity or Demon, emerged from the Ancient Tomb Sect as a third-generation disciple under its founder, Lin Chaoying. She trained alongside her junior fellow disciple, Xiaolongnü, under Lin Chaoying's maid. Mastering most of the sect's martial arts except the Jade Maiden Heart Sutra, she developed exceptional lightness skills and deadly proficiency with long, flexible weapons like the fly whisk, capable of inflicting severe injuries or disarming opponents. Her signature techniques featured the self-invented Divine Serpent Palm, emitting venom extracted from five poisonous creatures, with antidotes requiring her Five Poison Codex; the Silver Needles of Freezing Soul, needles tipped with lethal ice poison; and the vicious Triple Non-existing Hands, targeting vulnerable body points. She also wielded the Cold Moon Sword technique, a deceptive Ancient Tomb skill exploiting defensive openings.

In her youth, Li Mochou defied her master, leaving the sect to pursue a romance with Lu Zhanyuan. After Lu married He Yuanjun instead, a powerful monk intervened, forcing Li Mochou to delay vengeance for ten years. This rejection ignited her descent into merciless cruelty. She became a Taoist nun, adopting the epithet "Red Serpent Deity," though her youthful appearance sometimes earned her the moniker "Serpent Deity." Her notoriety grew from indiscriminate killings, especially targeting individuals named Lu or He, culminating in the massacre of the Lu family years after Lu Zhanyuan and his wife died. From this massacre, she abducted Lu Zhanyuan's niece, Lu Wushuang, as her disciple. Another disciple, Hong Lingbo, assisted Li Mochou in her schemes.

Li Mochou relentlessly pursued Xiaolongnü and Yang Guo to seize the Jade Maiden Heart Manual. After Hong Lingbo failed to secure the manual, Li Mochou continued her vendetta, cornering the protagonists repeatedly. Glimmers of conflicted nature surfaced occasionally, such as when she gently fanned the infant Guo Xiang with her fly whisk, but her obsession with vengeance and the manual dominated. During the events at Passionless Valley, Li Mochou encountered the poisonous Passion Flowers. Afflicted by delirium from the toxin, she hallucinated an image of Lu Zhanyuan within a raging fire and walked into the flames, meeting her end.

Her martial prowess initially ranked among the strongest, though she was later outmatched by Huang Rong and ultimately by Xiaolongnü, who mastered the Jade Maiden Heart Sutra. As Xiaolongnü's senior disciple, Li Mochou's relationship with the protagonists blended deep enmity with sect ties, complicating their conflicts.