Nanaka Kirisato is a serious 17-year-old girl whose main goal in life is to study hard and grow up quickly. She doesn't have any friends, except for Nenji, whom she has known since her childhood. Although she constantly lectures him about his immaturity and fighting, he is the only one that stays by her side. One day after an argument with Nenji, Nanaka falls down a flight of stairs causing her to lose eleven years of her memories. Now she believes that she is a 6-year-old girl! Nenji decides to protect Nanaka by keeping her condition a secret so she can continue attending school and lead a normal life. With Nanaka’s childish personality and her unstable medical condition, how long will Nenji and Nanaka be able to keep up this charade?
Sawamura Seiji is the most notorious delinquent at his high school -- always getting into fights and causing problems, leaving him single and unlucky with the ladies. After another long day of rejection, Seiji discovers that the love of his life might have finally appeared, but in the unlikeliest of places... his right hand?! Now, in addition to dealing with daily fights and the gossip of his classmates, Seiji must handle a new problem of embarassing proportions: Kasugano Midori, a pretty young girl in place of his hand, who happens to have fallen in love with him!
Both series are comedies with a little drama thrown in. In Midori Days and Nanaka 6/17 the lead male character gets into fights constantly and he also tries to take care of the female lead in one way or another. Also, both of the females have an illness that they are trying to overcome to get back to normal. When both of the females are ill (Nanaka having the mental capacity of a six year old and Midori being transformed into a right hand) they have the complete opposite personality of what they had when they were normal. They turn into talkative, outgoing, and somewhat childish girls. The main difference between the two anime is that Midori Days is very ecchi, while Nanaka 6/17 is not. I'm sure that if you liked one, you should give the other a try.