It is interesting to note that in Japanese, 18-ban (read as 'Ohako') and can mean favorite. It is also interesting that Seipai was Sensei's Miyagi's favorite Kata.
Seipai includes 'Gyaku-Waza' and 'Nage-Waza' with 'sekkin-sen' (close distance fighting)
Seipai has particular Goju movements called 'En' which means that all the movements are circular.
Seipai has a variety of 'Atemi-Waza' (strikeing techniques), 'Gyaku-Waza' (reverse techniques) and 'Nage-Waza' (throwing techniques). There is a good balance of techniques and kinetic motion, and is therefore a difficult kata to perform.
Sepai is found in Monk Boxing. The reference to '18' in naming this Kata has a couple of interpretations. Like Sanseru, there is a connection to Buddhist philosophy. Another explanation is '18 guards for the King'. The most apparent and most meaningful in the naming of Sepai is again from the martial arts development and the use of attacking pressure points. 18 is half of 36 suggesting that an alternative set of attacks and defenses of techniques and strategies from the original Sanseru 36.
There are numerous different theories, one suggestion out of many is that it is possibly derived from 6 x 3; the 6 represents color, voice, taste, smell, touch, and justice the same as the second 6 of Sanseru. The 3 represent good, bad and peace.
The name Seipai may originate from '18 (Seipai) Rakan-Te'. The kata does consists of 18 basic hand techniques of the 18 Rakau-Keu of Nan-Pa (Ha) Shorin-Ken. It is the 18 hands of basics from Tsuki. Geri and Uke (punch, kick and block).Rakan was a saint whose worldly passions attained a higher perception of life. Gautama (Sakyamuni) appointed the names of 18 saints and they reached the state of Arakan.
After Seipai, Sanseiru ( 36 hands) and Suparinpei (108 hands) was created. It is thought the two katas were created as a reverse, to show opposite and throw techniques of Seipai. This is thought to be why the numbers of Sanseiryu and Suparimpei are multiples of 18.