In chapter 13, it's revealed that Usuya's been secretly dating Kaori. Meaning, he's succeeded where the other guys at Tenbi have failed — by hooking up with one of the female students, and she's an upperclassmen at that!
In chapter 39, Takeru found out Usui has a foot fetish. In hindsight, his attraction to Himegami makes sense, given that it's one of the main fanservice tropes she caters to.
Ouken delivers a Wham Line at the end of chapter 49, when he asks why Takeru and his friends were trying so hard to protect Himegami. It initially comes across as evil not comprehending good, except that wasn't what he was getting at. The fact was: the girl they were trying so desperately to protect was already dead and had been for over 2,000 years. When viewed in that context, the Villain Had a Point.
During the flashback arc in chapter 51, Yatsuno told Mikoto the origin of the original 8 Maken and the shikigami:
Yatsuno: (to Mikoto) "Because I have been reincarnated seven times, I have been endowed with eight Maken. Incidentally, I can summon my previous lives as shikigami."
Which explains why Yasakani has the power of all 8 Maken combined and why Himegami can also summon the shikigami, since she's her biological daughter.
In episode 7 (season 2), Rudolf tells Himegami about his previous owner: a young girl, whose village was subjected to an Allied bombing run, during World War II. The final shot of the sequence shows the girl's bruised hand rising weakly admidst the charred ruins of her home, while Rudolf could only watch as she died.