I recall reading this book (in Spanish, don't remember if it was a translation or originally written in Spanish) around 2005, but most likely older than that. It was aimed at early teenagers, if I recall correctly.
The details are vague, but I remember there was this kid who was mute (if I recall correctly, he didn't have a tongue), and I think he was in foster care.
The main character was a girl, I think her family adopted the boy. There were suspicions that he might not be human, though I can't remember if this was because he had supernatural abilities (telepathy?) or because of his background. He was socially awkward and I think he confessed he was an alien to the main character, who struggled to believe him.
In the end, he disappears, and I think it is implied he wasn't lying (maybe even more than implied).
This I am not sure about at all, but I recall something being mentioned about the mute kid having been part of a circus.
The main character has a younger foster sister, who is kidnapped after school. They initially suspect the sister's real mother, who has caused nothing but trouble.
The alien child who can read minds helps resolve the kidnapping.
The alien child teaches the main character about his people, and how they live very isolated lives and do not interfere with each other or let humans know they exist. He also teaches her about anger and understanding others as well.
Once the alien child is going to leave, he tells the main character that she will forget him, and initially she seems to believe that he wasn't really an alien, until a few weeks later when the influence of whatever mind-erasing powers the aliens have wears off.