Elon Musk's brother Kimbal called Amber Heard "toxic" in the biography, out Tuesday
In the book, out Tuesday, the SpaceX founder and Tesla CEO, 52, and the Aquaman and The Lost Kingdom actress, 37, open up about their romance, with which they went official in April 2017. The pair dated on and off in the months that followed, before breaking up for good in 2018.
Isaacson writes of their relationship in the biography's "Rocky Relationships" chapter that Musk first took an interest in the actress after seeing her in 2013's Machete Kills, and that they met when she visited SpaceX a year later.
"I guess I could be called a geek for someone who can also be called a hot chick," Heard said in the book, while Isaacson claimed Musk took her for a ride in a Tesla during that SpaceX visit.
Among other tidbits about their dynamic, Isaacson said Musk flew to Australia in April 2017 to visit Heard while she was filming Aquaman, and that she once commissioned a costume to "roleplay" a character Musk said she reminded him of: Mercy, from the video game Overwatch.
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Isaacson also gets into some of the more tumultuous moments of Heard and Musk's time together, including a fight they had near the end of their relationship while on vacation in South America in December 2017 with family, including his brother Kimbal Musk.
While Heard allegedly admitted to Isaacson that the argument occurred, she also shared photos and videos of their evening afterward with him, which Isaacson said depicted the actress and tech mogul sharing a New Year's Eve kiss.
Of their relationship, Musk said in the biography, "It was brutal." For her part, Heard told Isaacson that she still loves her ex "very much," and added: "Elon loves fire, and sometimes it burns him."
Kimbal, 50, told Isaacson in the book that he thinks it's "really sad" his brother "falls in love with these people who are really mean to him."
"They're beautiful, no question, but they have a very dark side and Elon knows that they're toxic," he continued.
Asked by Isaacson why he falls for these types of people, Musk laughed and replied, "Because I'm just a fool for love. I am often a fool, but especially for love."
A rep for Heard did not immediately respond to PEOPLE's request for comment on Monday.
Over a year after the breakup, Heard told The Hollywood Reporter in December 2018, "Elon and I had a beautiful relationship, and we have a beautiful friendship now, one that was based on our core values. Intellectual curiosity, ideas and conversation, a shared love for science. We just bonded on a lot of things that speak to who I am on the inside. I have so much respect for him."
Elon Musk, out Tuesday, is available for preorder now.
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