Tefanus Kuusinen, or "Teppo," as he is called by friends, was the second-born son of half-blood parents and the younger brother of Aatu Kuusinen. Tefanus attended Durmstrang in his youth, and shortly after graduating, began teaching potions there, following a glowing reccomendation from the previous, retiring potions professor. Following the death of his brother, Aatu, and their close family friends, Kostantin Volkov and Ludmila Volkova, Tefanus became the legal guardian of their daughter, Yevfrosinya Konstantinovna Volkova. He is a very serious professor who is only well-liked by the students who've somehow developed a crush on him.

While Aatu attended Durmstrang, he and his best friend, Konstantin, were attacked by a werewolf. Luckily, they both survived the encounter, but they were permanently changed afterwards. Luckily, their grandmother was a master at potions, and, because Konstantin was quite wealthy, the Volkovs paid for the expensive ingredients, and the Kuusinens brewed the potion with enough mastery not to waste said expensive ingredients. Access to the Wolfsbane potion enabled both men to continue to go along with their lives safely. In order to help his brother, Tefanus also learned from his grandmother how to brew the potions, in case the need ever arouse or she was unavailable.

Regardless of the Wolfsbane potion and the fact that it made Aatu relatively harmless while in his werewolf form, he was killed by wizards who had heard accounts of a large beast in the local woods, and killed him due to anti-werewolf prejudice. The event was a tragedy for Tefanus, who deeply mourned the death of his older brother. Konstantin and his family traveled to Finland to attend the funeral, but due to a mishap, Konstantin misplaced his potion. When he transformed, his lack of control resulted in his attacking and turning his 12-year-old daughter, Yevfrosinya--a student of Tefanus' at Durmstrang, and his death, at the hands of his wife, Ludmila, who did the only thing she could do to save their daughter. Although Tefanus believed this tragic accident was the end of the pain their family-of-sorts was going to endure, Ludmila committed suicide not long after the affair. To Tefanus' shock, he was named in her will to take over as Yevfrosinya's legal guardian. Given that she had no other family to turn to in the event he declined and the fact that in general, he cared about her as a family friend and student, he accepted this new responsibility. Frosya moved to Finland to live with him, and Tefanus began brewing Wolfsbane potions for her so that she could continue to attend school.

Unbeknownst to him, Frosya developed a crush on him while living with him, given that compared to other adults, the two weren't very far apart in age. As she got older, she grew bolder about expressing this crush, eventually believing that it would be requited and openly confessing to him in her sixth year of school. Tefanus declined her advances (for numerous reasons, such as being her teacher, and viewing her as a sister), but her crush didn't pass. Frosya seemed to believe that once she graduated, there could be potential for them, and continued to try to convince Tefanus to give her a chance. This even occured when they were staying abroad at Hogwarts, where students witnessed Frosya's attempts and reported them, resulting in Tefanus' temporary suspension from teaching. Eventually, he was found to be innocent, and Frosya turned her attention elsewhere.