Kerr is one of the best forwards in world football and netted her 50th Women's Super League goal for Chelsea against Aston Villa earlier this month.
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That made her just the second foreign player in the WSL to reach that milestone.
WSL defenders have struggled to contain Kerr during her time at Chelsea, with the Australia international finishing as the league's top goal-scorer for the past two seasons.
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The forward also caused havoc during Australia's recent friendly against England, netting the opening goal in 2-0 win as the Lionesses slumped to their first defeat under Sarina Wiegman.
Kerr will be pivotal to Australia's chances of winning the World Cup on home turf this summer and for Chelsea teammate Charles, the 29-year-old is currently the best in the world.
Asked what it's like coming up against Kerr in training, Charles told talkSPORT: "It's a fun challenge, because she's always at it. You know that you're pushing and you're going to improve playing with her. She's incredible.
"What she does, she's the best in the world at it. It's definitely a challenge, but it also makes you feel comfortable because if you're training against her every day, you know you can't get much better than that."
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If coming up against Kerr in training wasn't enough, Charles is also tasked with dealing with a Lionesses teammate.
James - the sister of Chelsea full-back Reece - moved to the Blues from Man United in 2021 but struggled with injury during her first year at Kingsmeadow.
However, the young forward has enjoyed a breakthrough season with the Blues this year - scoring seven goals in all competitions - and her form has meant she has become a regular in the England set-up.
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James scored her first goal for the Lionesses against South Korea in the Arnold Clark Cup and ahead of the World Cup this summer, Charles has warned rivals that there's still plenty more to come from the 21-year-old.
"She [James] is a unique talent," Charles finished.
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"I think her ability to keep the ball so close to her [is unique]. In games, two or three defenders try and get near her and they're not able to. It's a unique talent.
"We're very fortunate to have her. She's only just getting started as well, which is the amazing thing."