Job: Bandit and face-stealing detachment of Naraku

Circumstances in Fuedal Japan (spoilers!): Musou is the first attempt by Naraku to rid himself of Onigumo's heart. Musou is the collection of fragmented memories of the human thief, trying to find out his past and present. He strongly desires Kikyo and her reincarnation, Kagome, but is unable to remember why. He is eventually eaten by Naraku after the half demon realizes that even with Musou's creation, he is still not free from Onigumo.

Naraku vs. Musou (spoilers!): Well, we already know the outcome of this unfortunate battle. Musou, despite having Onigumo's memories, is still just one of Naraku's children. When the man/youkai confronts Naraku, the poor demon is sorely over-powered, out-brained and just out-appendaged (by this I mean Naraku can grow more weird insect-y legs than Musou, who at one point attempts to imitate Big-Daddy Naraku by becoming a pseudo-scorpion). Also, Musou is easily distracted by the mention of Kikyo. He attacks Naraku when the half-demon mentions that Kikyo is nearby, and ends up being um...swallowed. Basically, in Naraku's grumbling about failed children, we realize that the half-demon was not able to completely purge himself of Onigumo, and therefore had no use for Musou.

Love Interests: since Musou is basically a mosaic of Onigumo's memories and personality, he is infatuated with Kikyo, and later her reincarnation, Kagome. What type of infatuation? Well, Musou developes a complex where he belives that the more he kills, the more he will remember. This includes slaying the women he thinks he loves. It's hard to judge whether Onigumo/Musou loves/hates Kikyo, but whatever he felt for her, it was obviously very powerful.

Note: Naraku later rids himself of Onigumo's heart by creating Hakudoushi. However, Hakudoushi is very different from Musou. This might have to do with his body- one of a boy, or it might be that all the unwanted parts of Onigumo were shed during Naraku's "big" transformation.