While Felton acknowledged that he's always harbored deep feelings for the 32-year-old actress, he explained that they haven't necessarily been romantic.
"I've always had a secret love for Emma, though not perhaps in the way that people might want to hear," he wrote in his memoir. "That isn't to say that there's never been a spark between us. There most definitely has, only at different times."
The "Flash" star admitted that he had learned that Watson had a crush on him when he was 15 and she was 12, but he had been dating someone else at the time.
"Rumors started to abound that there was more to our relationship than we were letting on. I denied that I liked her in that way, but the truth was different," he explained. "My girlfriend at the time knew straight away that there was something unspoken between us. I remember using the familiar old line, 'I love her like a sister.' But there was more to it than that."
Though their love for each other may not be romantic, Felton revealed that their relationship is tough to explain to others.
He continued, "I don't think I was ever in love with Emma, but I loved and admired her as a person in a way that I could never explain to anybody else… we were kindred spirits, I know for certain I'll always have Emma's back and she'll always have mine too."
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Watson also touched on their complex relationship and called him her "soulmate" in the foreword for his book that was leaked online.
"You know that person in your life who makes you feel seen? That person who is somehow a witness to all that unfolds? That person who knows—really knows—what is happening to you and what you're going through without anything having to be said?" Emma began. "For me, that person is Tom Felton."
Growing up, the "Little Women" actress explained that their relationship had started rocky.
"I was a moony and probably rather annoying nine-year-old girl who followed him round like a puppy, desperate for his attention. But, as he has written so eloquently, beautifully, and generously in this book, our friendship didn't end there," she confessed. "Thank goodness it blossomed and endured."
She shared Felton's sentiments and wrote that she "always struggle to explain to people the nature of our connection and relationship." Watson recalled friends hounding her for answers about their relationship and asking if the two had ever crossed the threshold from friends to lovers.
"I've lost count of the times people have said to me, 'You must have drunkenly made out, just once!' 'You must have kissed!' 'There must be something!' But what we have is far deeper than that. It’s one of the purest loves I can think of," Emma continued. "We're soulmates, and we've always had each other's backs. I know we always will."
"It makes me emotional to think about it," she said in the excerpt. "That's true friendship, and to be seen and loved like that is one of the great gifts of my life."