He once called her the big sister he never had but, more recently, with rumours that Prince Harry and his new wife Meghan, Duchess of Sussex are embroiled in a feud with Duchess Catherine and her husband Prince William, there's been talk the two warring couples have distanced themselves.
But now, in an about turn, it appears Harry has extended an olive branch to Kate – at a time when he needs her advice and support more than ever as he deals with his ongoing marriage dramas with Meghan.
Notably, Harry joined his sister-in-law at an Anzac Day service in London on April 25 where he was spotted gazing at the Duchess warmly.
Later he could be seen urging Duchess Catherine to walk ahead of him, in a gentlemanly manner, as they left.
WATCH: Prince Harry and Duchess Catherine's joint appearance on ANZAC Day. Post continues after the video...
"With William in New Zealand, it was the perfect opportunity for Harry and Kate to spend some time together," a palace aide tells Woman's Day.
"They've always had a great connection and Harry would do anything to safeguard that. This was his way of apologising for everything that's happened and showing his sister-in-law he's still standing by her."
Harry was not scheduled to attend the Anzac Day ceremony but decided to accompany Kate, 37, at the last minute – and looked thrilled to be doing so.
Their bond is well-documented and before he met Meghan, Harry used to regularly go over to Kate and William's apartment, where his sister-in-law would cook him roast chicken.
"Kate knows William and Harry used to be incredibly close and she wants them to have that bond again. She's trying to play peacemaker," explains our source.
"But that's easier said than done. Things are more frosty between the princes."
Harry's appearance at an Easter church service was telling – a body language expert claimed he was "anxious" and "desperate to avoid" his older brother.
"But Harry has shown he's not going to let this destroy what he has with Kate," our insider spills.
"He'll hate that their relationship has suffered and this public apology could be the first step towards ending this royal drama for good."