Invisible. The Aerling Series, #1

Olivia's best friend is not imaginary. He's not a ghost, either. And she's pretty sure he's not a hallucination. He's just Mason. He is, however, invisible. When five-year-old Olivia spotted the crying little boy on her front porch, she had no idea she was the only one who could see him. Twelve years later, when new-girl Robin casually introduces herself to Mason in the high school parking lot, they are both stunned.
While Mason is thrilled to be seen, suspicion and jealousy make Olivia balk at welcoming Robin into their circle. Fear that Robin's intentions aren't what they appear to be seem to be validated when a strange black car shows up outside Olivia's house. Olivia is determined to prove Robin can't be trusted, but can she find the answers in time to protect Mason from whatever secret threats are looming? Invisibility can only hide Mason for so long.