Onigumo

Onigumo was a thief who was rescued by Kikyo.Onigumo gave up his human body to demons in exchange for their strength. This created the evil half demon Naraku who then immediately tricked Inuyasha and Kikyo into becoming enemies and unwittingly caused Kikyo's death thinking she would use the Shikon Jewel to save herself.

Muso
When Naraku removed his human heart, he also removed Onigumo's soul in the process. Much later, Muso finally remembers about his past as a bandit and of his time with Kikyo within the cave.

Reuniting with Naraku
Realizing he was not yet ready to part with Onigumo's soul, Naraku confronted Muso and tried to absorb him back into his body. Inuyasha interferred, enabling Muso to run, but he foolishly went back to kill Naraku for revenge. Muso impaled Naraku but Naraku drew him in and absorbed him as he screamed for Kikyo, Kagura looking on as she thought, "If I'm not careful, that will be my ultimate fate..."

Final Death
In one of Naraku's final moments he recalled that Kagome said the the jewel did not grant his real wish. Naraku recalls that what he really wished for was Kikyo'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 Kikyo has gone. It is presumed that Onigumo's emotions have somehow surfaced within Naraku as he recalled his true wish.

Trivia

 * Even though Naraku was the ruling personality of Onigumo's former body, Naraku seemed to retained some of Onigumo's memories, and desires as well: The Shikon Jewel, and the priestess Kikyo.