Migi is the Parasite which lives in Shin'ichi's hand. Unlike "successful" Parasites, Migi has no desire to kill humans for sustenance, and is nourished by the food Shinichi eats. 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 he 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 can be reasoned with, however, and has just as much reason to be mistrustful of other Parasytes as does Shinichi. On the other hand, unlike Shinichi, Migi has no inclination to place himself at risk in order to protect other humans from Parasites. As the series goes along however, he becomes more human, whereas the reverse happens to Shinichi, up until an event involving a baby occurs.

(Source: Wikipedia)