7 Billion Needles

Koubu Hikaru is a lonely high-school girl who avoids contact with people and who always wears headphones.
One day, after being hit by a high temperature substance that fell from the sky, she loses consciousness. When she wakes up, Hikaru discovers that she isn't "alone" anymore: inside her body, Tengai, an alien plasma-based life-form, is now living symbiotically.
He is chasing Maelstrom, a being disguised as a human, who constantly commits massacres. If Tengai cannot identify him among 7 billion Earthlings, all of mankind will be doomed.