Katsui 【且井】3 The Japanese surnames etymology dictionary.
Tochigi-ken . Creation . Tochigi-ken Nikkou-shi Doctor Masaru Yoshihara in the Seogawa at the end of the Edo period tells that he treated the disease of Daimyo who visited Nikko Toshogu in Tochigi-ken Nikkou-shi Yamanoue and gave the character of "upper". There are also stories saying that in the same place they also named the surrounding residents when Meiji New surname was given. 吉原 reference.