Uwu means unhappy without you, it’s used in most social media’s, mainly in bios or even when tagging your significant other. It’s just another way of saying my one and only.
UwU is an emoji like XD, :D, ;D BUT...you can use UwU for everything for example:
A:hey i broke my phone
B:Thats bad ūwū
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A:hey i got A+ from my exam!
B:Great ûwû
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A:Fuck you!
B:UwU