He is his lord and master. Souei believes that Rimuru installing him as a ninja has unlocked his full potential and he is forever grateful. He and no other has a literal connection to Rimuru via a steel thread that allows the two to communicate via an unbreachable analog line.
Benimaru and Souei are rivals and best friends. For Souei, Benimaru who is the son of the lord was not his lord. Similar of age, Souei treats him as a rival. Someday, he considered to serve under him, but it was not meant to be. Instead, they began to serve a lord named Rimuru. Due to their close affiliation, he has acquired the blessing of Fire Lord's Shadow.
Souei and Shion seem to get along and are friends.
Souei and Shuna seem to get along and are friends.
Souka has the utmost respect for Souei and may even have the beginnings of a crush. She is very attentive to his instructions and looks for any opportunity to impress him. She competes with Grenda for Souei's attention.
Souei sees her as a nuisance. Due to Souei's testing his torture techniques on her, she has become infatuated with him. She competes with Souka for his attention.