"The Core Nine"
It is said that Wado originally had nine kata (Niseishi is not one of them), and that six were added later to the list that was presented in 1939 to the Butokukai in Japan (Japanese Budo organisation to register Ryuha=styles).
The original list from 1939 included Pinan Shodan, Pinan Nidan, Pinan Sandan, Pinan Yondan, Pinan Godan, Kushanku, Naihanchi, Seishan and Chinto. Between 1942 and 1945 six more kata were added: Rohai, Wanshu, Jitte, Bassai, Jion and of course Niseishi.
It is also said that Suparimpei was among the added Kata. True or not, that Kata is lost in time..
Here's news we got from JKF Wadokai officially in 2015...
Although all Kata have names and not numbers, the first nine Kata are usually practiced in sequence (with the exception of Pinan Shodan/Nidan/Sandan due to their different level of difficulties), with special importance to Kushanku, Naihanchi and Seishan, followed by Chinto. The Kata that were added later however, do not really have a specific order and you might question their importance to learning the Wado style.
Present day, JKF Wadokai has published a new book "Wadokai Textbook" which shows 10 Kata. It shows the original nine and Niseishi is included as well. During the official technical seminar of JKF Wadokai in Nagoya, August 2015, they explained that they include Niseishi "among the nine" (so now 10) because of its specific Wado characteristics.
During this seminar I emphasise the proper use of the hip joint. Usually you hear "twist your hip" and that's good AND you can go one step further to see what kind of dynamics can be used if you consider a left and right hip. Dropping the hip and controlling your hip socket. It's all there in the recording of the seminar on Niseishi.