he overhears Mizuki secretly telling Yuujirou that she is a girl
Sano meets Mizuki at the furo bathhouse
Sano walks into the bathroom while Mizuki is taking a shower
Mizuki's brother, Shizuki, comes to Japan and tries to take Mizuki back saying that she is a girl and can't attend a boys' school

2) Who's is Nakatsu's roomate? (gosh, this one is a giveaway, but I just had to add it)

forbidden secret lover, the gay doctor
Nakatsu has no roomate, and he's very very lonely
yogamaster Kayashima
womanizing older Nanba

3) How would Sano most likely answer the question, "What type of girl do you like?" as phrased by Mizuki?

"well...it's definitely not you."
"... I like someone who is always there for me, and someone I could depend on."
"........................ go to bed."
"No question about that, baby, your type."

4) After a fight, Sano almost all the time, gives Mizuki what treat?

a kiss to make-up after the fight, of course
a refreshing peach drink
take-out food to fill the body and mind
a big box of cherry-filled chocolates

5) One thing that both Sano and Mizuki's dad have in common is:

disgust of sweet foods
birthdays
sense of humor
love of furo baths

6) The main reason why Nakatsu can't date a girl is because:

he gets too nervous around girls, and never has a chance to talk to them
of course he wouldn't, he's gay
he's reserving himself for Mizuki
no girl wants him because he's plain ugly

7) Who was Mizuki's old flame?

Kagurazuka, Sano's high-jumping rival
Nanba, the charming dorm's upperclassman
Gil, her older friend in America
Shin, Sano's younger brother

8) When Mizuki goes back to America in Volume 12, Sano and Nakatsu also accidentally come to America, in San Francisco. They meet Mizuki's parents, and her parents insist that the two boys sleep over. How does Mizuki prevent the two boys from finding out she is a girl, when her parents don't know she attends an all BOYS' school?

she beats Sano and Nakatsu with a stick and knock them out until the night is through
she tells her parents to only use English because the boys are here to learn English, assuming that Sano and Nakatsu don't know English(of course, Sano does)
she makes loud, animal noises when her dad is talking about his DAUGHTER
she has her friend Julia help her distract Sano and Nakatsu when her parents are talking

9) Who does Umeda hate most of all?

Nakatsu, his heart-broken lover
Io-san, the woman who ordered him to work at her summer house
Nanba, his womanly nephew, whom he hoped not to be related to
Akiha, the exotic photographer

10) What sport did Sano inspire Mizuki to excel in?

tennis
soccer
high-jumping
track and field

11) Who did Nanba love during his adolescence(when he had shorter hair)?

his tutor, who helped him get into Ohsaka
a magazine woman on PLAYBOY
one of his hot school teachers
school classmate who rejected him

12) Why doesn't Sano want his brother, Shin, to also high-jump?

he hates Shin, and wants him to be miserable, and not have the joy of jumping
Sano is afriad his brother will out-jump him
he is worried that Shin will be pushed to hard by his coach, Sano's father
he thinks high-jumping is too dangerous for his brother because he cares too much