Mizuki Ashiya. Our heroine surely sees Umeda-sensei as a friend. She always comes to him when problems occur and he tries to help her... (for example by giving her pictures of him.. naked ^_^ ho ho ho)
Akiha Hara. A photographer. He is one year younger than Umeda-sensei and also went to Osaka High, therefore he calls him 'senpai'. Although he was married, he also likes Umeda-sensei -a lot-, who doesn't seem to be too happy about that.. but sometimes he just can't resist.
Ryouichi Kijima. He played a big role when Umeda was still a student and about 15 years old. Ryouichi always seemed to hide something (or his true self), which intrigued Umeda and he tried to find out more about him... For more, check out chapter 77-79 in vol 14.
Rio Umeda. Rio attends the St Blossoms High School (an all-girls school) and is Umeda-sensei's younger sister. She is good friends with Mizuki and also helps her (for example by lending her clothes..).
Io Nanba. Io is Umeda-sensei's elder sister and Minami's mother. Umeda-sensei is sort of scared or her and often hides when she comes to visit him. She owns an inn near the sea and also knows about Mizuki's secret - and helps her of course.
Minami Nanba. Io is his mother, therefore Umeda-sensei is his uncle and Rio his aunt. We can see him together with Umeda-sensei from time to time, so their relationship is quite good. He also is dorm leader at Osaka High.
Seira Umeda. Umeda's, Rio's and Io's mother. Minami's grandmother ^_^ Actually 51 years old, but very small and cute, Mizuki first thinks she is Rio's sister. Deeply in love with her (very handsome) husband Azuma.
Azuma Umeda. The father of the family, 54 years old. I can't recall if we actually see Umeda-sensei together with his parents in the manga. Anyway, everybody seems to be equally shocked that Umeda-sensei has such handsome parents. ^__^