It is never shown in the anime how or when the fairies started living with Sensei-san. All that was revealed was that they started living with him about a month before the series began. Even though the thought of "fairies" living in your house might be strange to others, Sensei-san takes it as a normal, everyday thing. He talks to them as if they're human and takes them to festivals and beaches, like they're all one big happy family. They even seem more like a family since whenever Sensei-san talks to them, he says, "Hello, my little girls, how are you today?" treating them as if they were his daughters. He obviously cares deeply for them and is very protective of them. He even tells them to not leave the house on their own and to come home at a reasonable hour if they're going to visit Tama-chan. Sensei-san also tries to give them as normal of a life as he can. So he gives them extra outfits to wear so they don't just have to be in their fairy outfits, and he introduces them to festivals and other things so they won't feel left out as much.
It seemed that Sensei-san is actually either a lecturer, TA, or researcher in some college or highschool. In the last episode, he is seen working in his lab alone.
Because Sensei-san either works or takes classes at the school, he is only seen at the very beginning and the very end of episodes, before and after school that day, not giving him a lot of screen time. However, the girls are very attached to him, and spend all their time with him on holidays when he doesn't have to go to school.Sensei-san is seen as "cute" and "handsome" throughout the show. He is even given a love letter in the June episode from a girl at his college. The four fairies got really excited and made the assumption that he was going to marry her and that she would be a June bride. However, at the end of the episode when Sensei-san came home, he told the fairies that he politely declined the girl because he had no feelings for her.
Even though Sensei-san has no feelings for the girls at his school, the four fairies he takes care of have feelings for him. Throughout the series, the girls are seen blushing whenever they think of him, and try to do anything to make him happy. In the February episode, the girls are determined to make the perfect chocolate for him, and spend the entire day trying to do so. However, the girls have trouble actually giving him the chocolate because of their nervousness. Luckily, Sensei-san eventually found the chocolate and asked them if their made it. He then ate it and praised them for making such delicious chocolate. Later, in the last episode, the girls had to deliever Sensei-san's "bento" (lunchbox) to his university when he accidently left it at home. They searched all over campus for him and had fantasies about him confessing his love to him along the way. Finally, they delievered his lunchbox to him and he gratefully accepted it. However, no couple was ever formed at the end of the series.
Obviously, Sensei-san is not his real name. The girls only call him Sensei-san because he is their "teacher." Even in the theme song, one of the lyrics was "Ah, teach us, Sensei-san!" The girls even thought that was his real name, which caused confusion for other people when they asked for him. For example, in the last episode when they had to go to Sensei-san's college, they asked one of the students where "Sensei-san" was. The girl looked at them, confused, and said, "Sensei-san? Do you mean your professor?" The fairies became confused because they thought that Sensei-san was his name. Unfortunately, his name was never revealed.
Sensei-san's next door neighbor and close friend is Tama-chan. Although Tama-chan is only a first grader and Sensei-san is at least a college student, they're very close friends and share a strong bond. Tama-chan is even seen hanging around his house a lot, and somehow knows how to get in even when he's not there. Tama-chan even hangs out with him at festivals and at New Years. It could be possible that they're childhood friends since Tama-chan's grandma has no problem with Tama-chan hanging out with Sensei-san. Their bond is actually so strong that she refers to Sensei-san as "Onii-chan," which means "Big Brother." However, they are not actually related.