Nikaido is a quite interesting character, but, unfortunately, he does not show up on the TV series. Many plots that Nikaido is involved are rewritten and Nikaido is mostly replaced by Yotsuya-san, which is interesting because Nikaido hates Yotsuya-san.
The walking insensitivity of room #2 is a college student who came from Ibaraki. He's always curious, so when he realized that nobody knew a thing about Yotsuya (everyone else had already given up on finding out anything about the peeping tom of room #4), he set out to find out about Yotsuya.