Simple. These are characters who end their sentences with “Na no da” instead of “Desu” or “Na” etc. Here are some examples of characters who say "na no da" at the end of their sentences frequently.
I really like this character type, they're usually very charming, cute characters. Niconico Pedia has an arcticle about these characters, along with a list of more. The page is all in Japanese though, but by searching the character names you should be able to figure out the characters, if not, I'll be adding more to the site soon.