The Australian cricket captain, who previously had a high-profile relationship with swimwear model Lara Bingle before their split in 2010, has reportedly changed since meeting Boldy last year.
A friend of Clarke told Woman's Day magazine: "People call Kyly the stunning enigma - and the mystery is all Michael's doing. He pushes that, as he's so scarred after the very public and drawn-out break-up with Lara.
"He's fanatical now about keeping what he and Kyly have together private for his own sanity. She doesn't mind because she's shy."
Describing Bingle as a "party animal", the source also said that Boldy is more "reserved" and "rarely goes out" with Clarke but added that he has become a better person since being with her.
"She's changed him, toned him down in imperceptible ways," they said. "She didn't like the glitz, which is why he seems to have matured and moved past all that. She's normal and nice and she made him see he really didn't have to try so hard."
The friend insisted that there is "no possible way" that Boldy sees Bingle as "a threat", saying: "Michael has made sure about that. Early in their relationship it was all they talked about, as he had a lot to get off his chest. They are rock solid behind their wall of privacy and even Lara's best efforts can't shake it all down."
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