So I was curious about what Nyanchu actually was and looked up the history behind this weird cat mascot that Ryuji’s always being compared to.
「ネズミの着ぐるみを着たネコ」という設定のキャラクター。ネズミの着ぐるみは「ネズミの気持ちを理解したい」という心意気の表れから。設定年齢は5歳。一人称は「ミー(me)」。言葉の最後に「ニャ」をよく付ける。また、「なんと」などの言葉は「にゃんと」になる。そのキャラクターとダミ声のギャップが特徴的。お饅頭が好物で、水が苦手。弱点は、フードを取るとくしゃみが止まらなくなること。
Nyanchu is an NHK television children’s character released in April 1992.
The concept behind the character is a “cat wearing a mouse costume”. With this costume they want to express the idea that “he wants to understand a mouse’s feelings”. Nyanchu’s age is 5. He uses “me” (in the English) to say “me” (instead of boku or watashi and so on), and often adds “nya” to the end of his sentences. He would also say “nyanto” instead of “nanto”. The ‘gap’ between Nyanchu’s character and his voice is quite distinctive. He likes steamed buns, and is bad with water. His weak point is that if you take off his hood he won’t stop sneezing.
source wikipedia (my translation)