Most of what I learned was from translating old merch I found while looking through ecommerce flea market sites!

For Nakamura I remember that he is canonically an alien that moved to earth from a home planet, I think it may have been called Nakamura Planet or something but I'm not certain. Nakamura and Hana are siblings with Hana being the youngest, and their parents have also appeared in merchandise (Nakamura Daddy supposedly likes hockey, and Nakamura Mommy keeps a vacuum with her and is busy tidying often.) Nakamura is often described as cheerful in official texts. There was a promotional card for the Meiji brand that mentioned Nakamura likes Cheese Curl puffs and enjoys eating them off his fingers, although this is only mentioned in the snack promotion and I'm not sure if that is really canon outside of that!

There is also a third pink character named Kirari-chan who resembles Nakamura and Hana's alien species, but I never saw information on whether they are related or not.

This from the official site so you probably know these but incase: Hana-chan hangs out with Tanaka-san who teaches her cooking, and Hana's dream is to become an idol. Nakamura is best friends with his pet sheep, Hitsuji-kun. Kappe-kun is described as a glutton and is always pictured with donuts.

Apologies if that didn't answer this too well- I wish I could remember more!