アルネ

A metallic life form that attached itself to a meteorite. In her original state, she resembles a large droplet of liquid metal. After coming into physical contact with Keiji, however, she adopts the form of a human girl by default. 

She can return to her 'slime' form if it's just fore few minutes, and she can even shape shift to imitate the things around her 
Like a baby duck, R-ne imprints on Keiji the moment they meet, and is thereafter always by his side. She sits on his lap and sleeps next to him whenever she gets the chance. 

Able to develop communication skills in just a few days, she demonstrates a frightening capacity to learn. While she doesn't feel "emotions" yet, they seem to show signs of awakening...