I think it's Kujyou because he is Karin's and Kazune's son.
Kujyou is a clone from kazuto kujyou, the "dad"
Other works by Koge-Donbo are Pita-Ten (which I highly recommend), Digi-Charat is cute, and Yoki Koto Kiku to name a few.
The name Karin is of Scandinavian origin and is a variation of the name Karen, which means "pure" or "beloved" in Danish. It is a popular name in Scandinavian countries and also has variations in other languages such as German (Karin) and Japanese (Karin).
As far as anyone knows, Karin doesn't have a last name, and neither do Juugo or Suigetsu.
Karin is a common name found in several different anime's, in Naruto Shippuden the 2nd season of the original anime Naruto, there exists a character named after Karin. In Kamichama Karin, the main protagonist is also named after Karin. As well as, Chibi Vampire the main protagonist is also named after Karin. Although, all three characters are named Karin, this does not necessarily mean that the meaning is all the same. Due to written form of romanji the names cannot be distinguished but if written in kanji form, the name meanings all differ as it depends on the combination of Chinese characters.
Karin Rodrigues's birth name is Karin Fehrmann.
Karin Resetarits's birth name is Karin Mller.
Karin Thome's birth name is Karin Ehret.
Karin Webb's birth name is Karin Payne.
Karin Falencki's birth name is Karin Tiche.
Karin Ahnborg's birth name is Ahnborg, Karin.
Karin Baal's birth name is Karin Blauermel.