Onigumo

was a bandit, rescued by the priestess named Kikyō, who fed his body to demons and ultimately became Naraku.

History
He was an evil human being who gained his injuries in a fire caused by Rasetsu who set the building Onigumo was in a blaze because Onigumo betrayed him and took his men. Kikyō found him and nursed his injuries, and occasionally, Kikyō's younger sister Kaede took care of him as well, even though she didn't like doing it; Onigumo had originally had difficulties accepting Kikyō's kindness, telling her to forget about the bandit when she visits him, but Kikyō's kindness had also made Onigumo fall for her. Onigumo gave up his body to demons in order to regain his strength, mobility, and power. He did this because he desired Kikyō and the Sacred Jewel for himself, while the demons did it because they wished to get rid of Kikyō and obtain the Sacred Jewel. This created the evil half-demon Naraku who then immediately tricked Inuyasha and Kikyō into becoming enemies so he could have the sacred jewel for even more power than he already had. His deceit led to Kikyō's death and Inuyasha's sealing to the Tree of Ages. When Naraku removed his human heart, he also removed Onigumo's soul in the process, which became Musō. Much later, Musō finally remembers about his past as a bandit and of his time with Kikyō within the cave.

Final Death
In one of Naraku's final moments, he recalled that Kagome said the Jewel did not grant his real wish. Naraku recalls that what he really wished for was Kikyō's love, but it seemed that it was not granted to him. Before Kagome's arrow pierced the jewel, Naraku comments that he won't be able to go where Kikyō has gone. It is presumed that Onigumo's emotions had somehow surfaced within Naraku as he recalled his true wish, thus proving what Kikyō once stated to Naraku was indeed true;"You've disguised yourself well, but you're still that fugitive human, and who you once were can never be erased".

Trivia

 * Onigumo had a Spider mark on his back, even before he became Naraku.
 * The name Onigumo literally translates to Ogre/Demon Spider, most likely referring to his demonic spider mark on his back, and possibly Naraku's spider-like traits.According to Japanese History, Japanese people used "Tsuchigumo" (ground spider) as a derogatory term for bandits and thieves
 * It is Naraku's actions had brought some sense of irony with Onigumo's words of how he would have liked to see Kikyo "apprehensive and frightened," a facial expression that was made as Naraku struck down Kikyo, as he was disguised as Inuyasha, though Muso, Onigumo's later reincarnated form, expresses a large amount of anger and regret as he recalled that bitter memory as Kikyo had died from such a mortal wound due to Naraku's miscalculation.
 * As Muso, while Onigumo did remember about the Shikon Jewel, he apparently did not seem to desire the jewel as much as he did with Kikyo which implied that Onigumo and saw the Shikon Jewel as merely a bonus and his main objective was Kikyo.