*might contain spoilers*
Migi is the Parasite which lives in Shinichi's right hand, named after the Japanese word for "right". Unlike "successful" Parasites, Migi has no desire to kill humans for sustenance, and is nourished by the food Shinichi eats. At first, Migi is like other Parasites, completely without emotion. His primary consideration is survival, and he has threatened and in some cases attempted, to kill other humans who pose a threat to his and Shinichi's secrecy. When he and Shinichi were first coming to terms, he even threatened to remove Shinichi's other limbs in order to render him unable to place the two of them in danger.
Migi gradually evolves over the course of the series, he becomes more human. Migi is rather different from other parasites, he is intelligent without any trace of bloodthirst, is very reasonable and has just as much reason to be mistrustful of other Parasites as does Shinichi, though he always says that only his own life matters to him, he has saved Shinichi and many other humans many a times. In the final battle with Gotou, Migi saves Shinichi's life putting his own life on the life. After the final battle with Gotou, having been absorbed by the Parasite prior to his defeat, Migi's composition is greatly altered to the point that he enters a deep sleep though he briefly woke up to save Satomi without Shinichi's realization.