One other thing apparent in the OP is an overflowing of cutesyness. This is going to be the main armament of the less equipped heroines. Fortunately it's treated by healthy appreciation on Youto's part, without melting away. Well, save for the customary going back to childhood-part, the barely hidden steamy reactions to each other's younger forms are hilarous.
Tsutsukakushi Tsukiko couples this charm with the acquired inability to express emotions, the hit representative of which is the disenchanted munching sound she makes when indulging in her favorite hobby -- sweets. She's going to be the girl always near the protagonist, perseverant but tempered.
Not like Azuki Azusa, for sure. Thinking back, her character seems not as well thought up as that of the others, I might go as far as stating that it is the weakest point of this show. She starts with the rôle of the haughty spoiled girl admired by all, but quickly falls for Youto, and after that she only appears as a mock image of affection gone overboard. Maybe all this was only a plot device to draw out Youto's gentler side.
Ah, Koutetsu-san! The elder sister of Tsukiko is certainly the stacked flagship of this series. She wears the objects of all the boob jokes in an unaffected way, which together with her utmost naïveté makes her an interesting character. Thankfully her extreme affection for Tsukiko is quickly subdued. That's done much better by her sidekick, with her boisterous perverted ranting.
All in all, this is a comedy with a merry atmosphere and unexpected dramatic tones, where all the drooling over female characters is made explicit and jokingly self-referential.
