This is a very serious question to which the fate of all humanity depends upon.
Everytime I open up UE I think for a second or two about whether the “U/u” is uppercase or lowercase. The serif on the bottom right makes me think it’s lower-case, but the tallness of it and the narrowness in between the two sides implies that it is uppercase.
I searched for duplicates for this question and found none.
I hope that you’re having a good day! That’s definitely the first time I’ve saw this question. It looks to be an uppercase ‘U’. I know that was a pretty short answer. But, I hope it helps!