Aoishiro
Akaiito vs Aoishiro
Warning! Contains spoilers for both games.
Hatoh Kei vs Osanai Syouko
The brave protagonists.
As a girl's love game, it's only natural that the protagonists are girls.
Their personalities are quite the opposite with Kei being a bit of a ditz (dojiko) even though her motto is to always be prepared. Syouko has a serious personality and a gruff manner. Syouko is also proficient with a bokutou while Kei relied on other's to help out when she was in trouble, which was quite a lot of the time ^__^
Kei's family DOES have talent for martial arts. Her mother was a famed onikiri and her twin brother Hakuka was also able to master difficult martial arts in a relatively short training period so perhaps Kei does have latent ability. Maybe she can join the kendo club in college?
There are similarities between both of them though. Both have memory loss regarding a part of their childhood. Both also experience an event in the summer that will change their lives. A mysterious person close to the protagonist is also in both games.
Both also have an older person that they admired in childhood. Kei has Yumei and Syouko has Natsu.
And last but definitely not least...all the other girls are attracted to the protagonists, whether in a platonic way, as food, or as more. But then again, this is a visual novel, so that's to be expected. :p
Senba Uduki vs Kyan Migiwa
The stylish onikiri.
Once again, a bit of opposite personalities. Senba resembles Syouko's calm personality while Migiwa resembles Sakuya's personality. I think Migiwa likes tweaking Syouko just as much as Sakuya likes tweaking Kei. However, when Migiwa goes into "onikiri-mode", then she is a cool, serious killer.
As onikiri, both are excellent fighters and possess weapons for that task. Uduki has her sword and Migiwa has her pole, which has a hidden blade contained within. Both also have a "power" in their right eye that enables them to detect oni and other spiritual creatures. Also, both of them have no compunction about carrying out their duties as onikiri even if it means slaying oni who have yet turned into oni.
Yumei vs Nami
The mysterious persons.
Their pasts and identities are unknown. In the beginning, both are only known by their first names written in katakana. Both are softspoken. Well, in Nami's case, she hasn't spoken at all, so maybe that doesn't count. They both dress in Japanese-style kimonos and they both even have butterfly motifs on the sleeves of their kimono. They also have a childhood connection to the protagonist.
Both are presumed to be non-human and have a ability to heal injuries. A little blood from the protagonist also seems to be a tasty snack for both.
Some differences are that Yumei is older than Kei and is a caretaker for her, while Nami is quite a bit younger than Syouko and needs taking care of.
Other
In both games, blood is an important aspect. Of course, since Kei possesses the "Nie no Chi", just about everyone is after her for it. Syouko's "energy" and perhaps her blood appears to have changed after touching the Tokosaki no Tsubaki. I wasn't expecting too many vampire scenes in Aoishiro besides maybe in Kaya's route, but it appears Yasumi had other thoughts. ^__^
Snakes are an important symbol in both games. In Akaiito, Nushi is symbolized by a snake and both Nozomi and Mikage used snakes in their attacks. The snake's ability to petrify prey is also used. In Aoishiro, Kurou-sama turns out to be a snake god and Migiwa's techniques also resemble a snake's.
The reference of the light from the moon being dead is used in Sakuya's route when Kei, Sakuya, and Tsuduru are watching the moon. It's also used by Syouko when she wakes up in the middle of the night. This is a bit of an interesting departure from usage in other places where the moon is bright and filled with life.