Age:appears to be 18
Species:hanyo; half dog demon
InuYasha is a brave but naive hanyo (half demon), the result of a relationship between a powerful dog demon and a beautiful human woman. Orphaned at a young age, InuYasha is rejected by humans because of his yokai (demon) side and looked down upon by yokai because his human blood supposedly taints his superior yokai blood. Thus, before InuYasha met, and consequently fell in love with, the powerful priestess, Kikyo, he found it very difficult to trust anyone else. However, InuYasha is a half demon, and therefore ages differently than a normal human.
Since the Jewel of Four Souls (Shikon No Tama or Sacred Jewel) was powerful enough for InuYasha to become a full yokai and Kikyo was the protector of the jewel, InuYasha was motivated to follow Kikyo. However, the two fell in love, and eventually, Kikyo talked InuYasha into becoming human, instead. A human, named Onigumo, plotted to take the jewel for himself. He was once a bandit who was obsessed with Kikyo and became jealous that she had fallen in love with InuYasha. Unable to take any other action, Onigumo gave his body to a horde of demons for power. Their union is Naraku. Naraku disguised himself first as InuYasha and then as Kikyo to deceive them into betraying each other. Enraged at Kikyo's "betrayal", InuYasha broke into Kikyo's village and stole the sacred jewel. To defend the village and to keep him from escaping with the jewel, Kikyo shot InuYasha with a sacred arrow pinning him to Goshinboku (tree of time). Moments later, Kikyo collapses from a wound she received from Naraku and gives final instructions to Kaede, her sister, to burn the jewel with her body so it would travel to the afterworld with her.
500 years later, Kagome Higurashi, a junior-high-school student, unknowingly carried the Jewel of the Four Souls within the left side of her torso. Kagome is the reincarnation of Kikyo. A centipede yokai called Mistress Centipede attacked Kagome and dragged her into the Bone Eater's Well which transported her back to the Feudal Era. She arrives Fifty years after Kikyo pinned InuYasha to the Goshinboku. In order to defeat the centipede, Kagome releases InuYasha. InuYasha is thinking how first, he will kill the centipede, then do the same to Kagome to get the jewel. After InuYasha kills Mistress Centipede, he tries to steal the Jewel but ends up with the Beads of Subjugation around his neck. When Kagome says "sit," the necklace plunges to the ground, taking InuYasha with it.
InuYasha becomes more civil as time progresses, losing some of his more childish qualities and becoming much more mature. He always tries to take the most direct route possible in solving any problem, which usually involves violence. He maintains a crass, rude attitude towards everyone except Kagome and Kikyo, and to a lesser degree the rest of his companions except Shippo. While one of InuYasha's character traits - which is pointed out by other characters - is his tendency to allow his anger to cloud his judgment and cause him to act rashly, we see him change over time such that he begins to care about Kagome, and it is clear that despite his original intent, presumably, that he would have killed her to get the jewel, we see that InuYasha would die, if necessary, to protect Kagome.
InuYasha is offended when someone calls him a mangy mutt, puppy, half breed, or otherwise suggests that he is inferior to yokai, due to his human parentage. Although InuYasha should be weaker than full demons since he is a hanyo, he tends to remain the victor in fights largely due to the fact he wields Tetsusaiga (a very powerful demon sword), and his heritage being from that of a dai-yokai (grand demon) thus allowing him to take a humanoid form like Sesshomaru, Naraku and Koga giving him a great deal of power.
Being a hanyo, InuYasha loses his demonic powers periodically and is turned into a normal human boy with black hair. For him, this "time of weakness" occurs during the first night of a new moon. His team members try their best to protect him at that time, and keep this vulnerability secret from others. Despite their efforts, however, some of InuYasha's adversaries, including Kagura, learn of this weakness. However, Kagura does not use it against him.