Yoshiyuki Terada is a teacher at Tomoeda Elementary School, serving as the homeroom teacher for the main characters during their fourth and fifth grades. He also teaches other subjects, including physical education, and accompanies students on several field trips. As the series unfolds, it is revealed that he is romantically involved with his student, Rika Sasaki. In the manga, they become engaged when he gives her a ring, hoping it will one day become her wedding ring. Rika gifts him a teddy bear she made and named after herself, believed to ensure their eternal love. Near the end of the series, while Sakura is uncertain about her feelings for Li, Terada and Rika are seen having a picnic in a park, with Terada slipping away so Sakura won't notice him. Throughout the series, none of the other characters seem aware of their relationship, though Sakura hears Rika call out "Sensei" when using the Illusion card to reveal her most beloved person.
In the anime adaptation, their relationship is portrayed differently. Their engagement and romantic scenes are omitted, and Rika notes that Terada reminds her of her absent father. Terada accepts cakes from Rika and names the bear after his "hardest working student." In the Cardcaptors English dub, he is referred to simply as Mr. Terada, and all aspects of the romantic relationship between Terada and Rika are removed. Rika's shyness and reactions towards him are depicted more as a general fear of men rather than romantic feelings.
