Wind: A Breath of Heart (TV)

both series have a recurring theme each: gift has a rainbow while wind has an airship.. plus both series are focused on romance with a little comedy, and both series have girls skilled with a sword: hikari tsukishiro and rinka hokazono, and have pretty blond haired ladies: nozomi and chisa fujimiya(hey! both characters have the same surname) 

both series featured the male protagonist returning to their hometown and are childhood friends with the female protagonists 

both series have the main male and female protagonists as childhood friends, both male protagonists makoto okano and wataru hoshino are good friends with the other female protagonists, and are both romance, drama and with a few humor and both protagonists makoto and wataru are friendly with all the female protagonists  

both series depicts the main protagonists as childhood friends that were separated for many years and reunited at the beginning of the series, and both nana aihara and hinata okano are both cheery, childish and energetic, and are both younger sisters to their onee-chans kazuki aihara and makoto okano  

both series are typical harem type anime series consisting of 6 girls plus one guy, with the male and female protagonists as a couple (makoto okano and minamo narukaze and takumu nishijou and rimi sakihata), and both male protagonists have one of the series main female characters as a sister (hinata okano and nanami nishijou), although unlike wind a breath of heart, the male and female leads of chaos head are not childhood friends 

These both start out as your typical 2004 harem anime, being all slice-of-life and happy and oh, look, a love triangle, how creative. However, all the sudden, BLAM supernatural. Then the show gets much darker.  

Both of these stories are originally created by minori. They look like a school slice of life with some elements of romance and drama. And by the way, Makoto Shinkai took part in creating these series. 

both series featured the main male and female protagonists as childhood friends plus both male protagonists Kenji Tomosaka and Makoto Okano have comical male partners Tsutomu Tachibana and Hatano, and Both Series' Main Protagonists have one of the female protagonists as a Sister(Makoto and hinata Okano and Kenji and Suzuka Tomosaka; both of the protagonist's younger sisters even calls the female protagonists as onii-chan or the japanese way of addressing an older sister or female), and also the male leads, their sisters and the female leads are all friends with each other 

both series are about romance and every episode have 12 minutes each. plus both the main protagonists have one of the female protagonists each as a sister makoto has hinata while ryo has akane 

both series have wierdo looking girls, osaka kasuga and hikari tsukishiro, although osaka is smiling while hikari is more serious, plus both series have cheery, childish girls chiyo mihama and hinata okano, and have long-haired, glasses wearing ladies, yomi mizuhara and kasumi shikohin. the difference though is that azumanga daioh is more comedic while wind a breath of heart is more on romance and drama with some humor thanks to tachibana. in some instances, makoto and tsutomu are like the male versions of yomi and tomi, with makoto being the male version of yomi and tsutomu, the male version of tomo 

both series' main protagonists have known each other since childhood, and have an object to remember a promise (sandbox and harmonica) plus have ladies skilled with a sword, hikari tsukishiro and motoko aoyama, and cheery and energetic girls kaolla su and hinata okano 

both series featured the heroines having powers and abilities, though wind a breath of heart is more on romance and drama (with humor thanks to tachibana) while ah my buddha is more on comedy.. also, hinata sugai and hikari tsukishiro are both the most silent types of both series.. plus have some similar characters: blond haired girls: haruka amanogawa and nozomi fujimiya, short haired and glasses wearing ladies wakaba fujimiya and sakura sugai 

both series' respective protagonists have met each other since childhood, separated and are reunited at the beginning of the series, and the females are in love with the male, and all the girls have special powers and abilities, although the difference is wind is the drama version while koi koi 7 is the comedy version 

both series featured perverted guys that often get beaten by girls: Tsutomu Tachibana and Kyosuke Imadori plus cheery type girls: hinata okano and tenma tsukamoto. plus yakumo and hikari have both adopted a cat, and have pretty blond haired ladies, eri sawachika and nozomi fujimiya