The Apprentice star takes over from presenter Jeff Stelling when the popular Channel 4 series returns next week.
Speaking to The Mirror, Hewer praised Riley and Dictionary Corner expert Susie Dent.
"Rachel is sharp as a whip. She's extraordinary," he said. "During the numbers game she will stare into space with her fingers moving and three seconds later she stops because she's got the answer. Unbelievable. Charming girl, wonderful. She's the definition of toothsome."
On Dent, Hewer added: "She is absolutely delightful and smart too."
However, the 67-year-old admitted that he was initially unsure about taking on the role, saying: "I just wanted to know if I'd be any good. I so want it to be a success. I still think I'm talking garbage, but they say it works. My style is a bit savvy, not unkind and not cocky."
Hewer revealed that on one occasion during filming, he had shouted out a word he found. "It was quite amusing, I looked up and shouted 'conifers'. Both contestants then gave the word 'conifers' themselves. I don't think they'll be able to show that on the telly. I was just so excited to have got it," he added.
Countdown returns to Channel 4 on Monday, January 9 at 3.10pm.
> 'Apprentice' Nick Hewer: 'I was nervous about Countdown role'