Sirius

• He uses cosmic terms to describe his emotions—for him, "sadness" is a "fading glow," and "falling in love" is "gravity pulling you toward you."
• He talks about the stars as brothers and misses the sky.
• Sometimes his body begins to emit a soft glow when he experiences strong emotions.

• teaches him how to use telephones, electricity, and the internet;
• explains why people cry, laugh, and get jealous;
• shows him what it's like to be alive and imperfect.

It turns out he's a fallen star who's taken on human form. He needs to learn to understand people, otherwise he'll dissolve—forget who he was.
The heroine decides to help him adapt to life on 21st-century Earth.