Let me know if you find a source that shows the character with the most mentions whose first name starts with a U. Urg the Unclean is the only one I've found so far, who is only mentioned by Ron once.
Not sure if you count characters whose first name is unknown, but Slytherin Quidditch captain Urquhart is mentioned twice in HBP. A number of other characters are mentioned once.
Petunia's friend Yvonne is mentioned once in Philosopher's Stone.
According to the source, Katie Bell is only mentioned 116 times, while Kreacher is mentioned 312 times. It's probably because Katie is mostly just mentioned when talking about the Quidditch team, while Kreacher was probably named frequently whenever they were at Grimmauld Place.
Just added Yvonne. I looked into Ulick Gamp, and although his portrait talks in the first chapter of Half-Blood Prince, he is never named. So unless he is named elsewhere in the books, I would rather not include him if his name isn't mentioned.
Petunia's friend Yvonne is mentioned once in Philosopher's Stone.
Sources: books & Harry Potter Wiki
As for Yvonne, I'll look into it and probably give it as a provided answer like Urg the Unclean.
Thanks for the suggestions!