Seven Billion Needles

Hikaru Takabe may not be the most social of teens. Always sporting her headphones, she gives off an aloof aura that rubs her classmates the wrong way. But her not being part of the crowd takes on a different dimension when she becomes involved in an intergalactic game of cat and mouse.

Inspired by Needle, the Golden Age hard sci-fi classic by the late Hal Clement, Nobuaki Tadano's debut work brings a unique take on alien invasion up to date and into the maelstrom that is the Japanese high school girl.

Tropes

 * Alien Invasion - The alien Maelstrom takes a host and then goes on a killing spree. Horizon follows him an attempt to stop him.
 * Heel Face Turn -
 * Headphones Equal Isolation: Hikaru.
 * Kuudere - Hikaru.
 * Lovecraftian Superpower - Hikaru's symbiosis with Horizon borders on this, but the trope really comes into play when.
 * Nice Job Breaking It Hero -
 * Ordinary High School Student - Hikaru, until she ended up sharing her body with a super-powered alien.
 * The Power of Friendship / The Power of Love - Or, as Horizon calls it, the "heart bonds of lifeforms" are what save Hikaru from Maelstrom's nightmare world.
 * Secret Keeper - Saya, though she doesn't know exactly what's up with Hikaru, knows that she's involved with some sort of craziness.
 * Strange Girl - Hikaru.
 * Tearful Smile - Hikaru, near the end of the second book.
 * Vicious Cycle - Maelstrom attempts to wipe out all life on a planet, Horizon chases him and attempts to stop him. They move onto another planet, Maelstrom wipes out all life, Horizon chases him, they move onto another planet...
 * You Are Not Alone - Said to Hikaru several times.