In Philippa Pierce's Tom's Midnight Garden, Tom is usually invisible to everyone except Hatty when he is in the garden. Let's look at why this is.

Tom seems to be going back into the past when he walks out the back door and into the garden in the middle of the night, as the old grandfather clock adds another hour to the day. He does not belong to the world he enters. He is not supposed to be there, and the people there cannot see him. Only Hatty can.

There is actually a very good reason why Hatty alone can see Tom. As the novel progresses, we learn that Hatty is still alive, even though she is not a little girl any more. She is now the elderly Mrs. Bartholomew, and she lives in a flat in the same house where Tom is staying with his aunt and uncle. She has been dreaming about her childhood, and she is lonely. Somehow her dreams have opened the door for Tom to cross over into the garden in and through those dreams. This is why Hatty can see him and interact with him. The lines of reality and imagination blur and fade, but two miserable people find each other and become friends for life.