A 23 year old half alien, half human, Planet Inspector from the Galactic Federation, sent to Earth to observe and study humans. Mizuho came to Earth to monitor the planet though inadvertently was seen by Kei as she was just landing on Earth. Although she did not trust him at first, she started to become attracted to Kei and eventually falls in love with him after they shared secrets about their past to one another. In a conversation with Maho, Kei learns that she is the youngest member of the GF Observers ever to gain a license. Also from Maho we learn that Mizuho apparently has never had a romantic relationship, which is verified when she admits as much to Kei.

One of the main conflicts in the story involves Mizuho and how she tends to have great jealousy and misjudgment. She even went to such great lengths as to spy on Kei while he was hanging out with a friend from school, Koishi Herikawa, in a set up date, and when Mizuho's mother held Kei hostage in a hotel.

She loves to eat pochy, a Japanese snack food that consists of a biscuit stick covered with chocolate. She eats Pochy because it reminds her of her dead father; an empty box of Pochy was all Mizuho has left of him (she actually throws away the boxes she buys and eats them out of that old box). Her mother and sister are also seen eating it in several episodes. Pochy is a fictional version of an actual snack known as pocky.