Locum Tenens

lo·cum te·nens (lō′kəm′ tē′nĕnz′, tĕn′ənz)
A person who temporarily fulfills the duties of another.

When young Alex's big brother, Mac, attempts suicide, he's sent away and Alex has to deal with his newfound loneliness as his best friend is no longer around. This is when Thalia, Alex's new best friend, enters the picture. She perfectly fills the spot in Alex's life where Mac once stood, giving him companionship and guidance when he needs it most. They become inseparable, and it isn't until Mac returns home from the hospital that she disappears.