Tom Long
Tom is the protagonist of the story. He is a likable boy who is in quarantine in Cambridge because Peter, with whom he is close, has the measles. Although Tom has shown now symptoms he is quarantined just in case. Tom is a gregarious boy who is bored and extremely lonely at his aunt and uncle's apartment. He is also fidgety because he has literally nothing to do all day and is not burning up any energy at all.
Tom is a child who is open to believing in the possibility of slipping back and forth through a wrinkle in time. He is delighted to find a playmate, albeit a Victorian one, because he has been so lonely, and this excitement overrides any nervousness or unease about traveling backwards and forward in time on a nightly basis. Tom genuinely likes Hatty and feels that a special bond is developing between them. He feels this connection even when they are no longer meeting in the garden and he sees in the elderly Mrs Bartholomew the person he has made friends with, which is why he gives her the kind of goodbye hug that he would give another child.
Hatty
Victorian Hatty lived in the house when it was still one big house, long before developers converted it into apartments. Hatty is lonely because there is nobody at the house for her to play with, or to do anything fun with at all. She seems to grow up very fast, far faster than Tom, but remains friends with him and seems to enjoy the magic of their meetings just as much as he does.
Hatty starts dating a young man called Barty and clearly thinks a great deal of him because she cannot wait to tell Tom about him. She is still young enough to connect with her younger self and does not seem to be in any particular hurry to grow up. She is still enthralled by magical experiences such as going ice skating in the garden with Tom on one of his visits.
Mrs Bartholomew
Mrs Bartholomew is two characters in the novel, the elderly lady who lives on the third floor of the converted house, and the young girl whom Tom meets in the Victorian era. As an older woman she is sometimes cantankerous and the other residents of the house seem to be quite wary of her, and anxious not to upset her with too much noise. Mrs Bartholomew misses her childhood and the way that things used to be, and dreams about the glory days of the house every night. Tom walks into the garden into whichever scenario she was dreaming about and so we know that each of these moments in her life was memorable and important.
Mrs Bartholomew seems to connect with Tom as an elderly lady in the same way that she was able to connect with him as a young girl in her dreams, and feels the same bond to Tom that he feels to her, because he brings out the youth in her that many feel she has forgotten all about.
Peter Long
Peter is Tom's brother and is pivotal to the story despite hardly appearing in it; because of his measles, Tom is sent to live with his aunt and uncle for a period of quarantine. He is present throughout the book as Tom's confidante and receives updates on what is happening after midnight in the garden from his brother, whose complete belief in the possibility of time travel he seems to share without question.
Aunt Gwen and Uncle Alan
Gwen and Alan take Tom in when he needs to go somewhere for quarantine. They don't really have the room in their apartment, or in their life, for a young boy, but obviously love their nephew and are both generous and lenient with him. Alan believes that Tom sleepwalks and uses this as the explanation for Tom flailing around outside and falling over the garbage cans. Aunt Gwen is far more observant than her husband and watches Tom's actions quite a lot, finding curiosity in the way in which he hugs Mrs Bartholomew goodbye, seeing something unusual but not being able to quite put her finger on it.
