With the year coming to an end, the circle of friends cannot find any kind of reason to celebrate, but a small catalyst takes action in order to bring everyone back together and push towards a full resolution.
Perfect December contains the following examples:
- Absurdly Sharp Blade: The sword Mayoi gives to Masaki before he enters Mikami's realm. It cuts Shirou's head off in one slice and destroys a god's barrier afterwards.
- Accidental Innuendo: In-Universe, several from Mizuho, all of which are Lampshaded by Kasumi.
- Because I Said So: What Shiro resorts to after Fuyu, her youngest daughter, defeats her in a logical argument during The Stinger.
- Big Brother Is Watching You: The wolves that patrol the town during this chapter are pretty much security cameras so Shiro can keep an eye on things from Mikami's realm.
- Book Ends: The series began with Masaki's thoughts as he arrived in Tagami and his meeting with Shiro. The series ends with Yuuki's thoughts as he arrives in Tagami and his meeting with Fuyu.
- Brick Joke: The doctor from the first chapter telling Masaki, "I know I told you we should meet again, but not in an ambulance!"
- Call-Back: A subtle one but how Masaki defeats Shirou mirrors how he defeated Yuki in the first chapter.
- Corner of Woe: Masaki during his Heroic BSoD as he sulks in the corner of Kouhei's room, watching one of his videos, and comparing the model to Yuki at every opportunity.
- Darkest Hour: Yuki and Shiro are gone, Sanae's leg is paralyzed, and the circle of friends has pretty much disbanded.
- Death Glare: Ao has a very epic one in a CG after Masaki had asked her to help him with something: taking her with him to buy an engagement ring for Yuki and using her finger to check the size.
- "Eureka!" Moment: Mizuho after hitting a dead end on how she can help bring everyone back together.
- Get a Hold of Yourself, Man!: Mizuho does this to Masaki to snap him out of his Heroic BSoD by drafting him into her town patrol and keeping him too occupied to be depressed.
- Hand Wave: Mayoi tells Masaki his sword will only work once but actually works twice, prompting Masaki to declare it, "True Japanese Quality."
- Heroic RRoD: Masaki nearly passing out from the pain once the adrenaline rush of defeating Shirou wears off and he's reminded that his right arm nearly got torn off. Happens a second time and he actually does pass out after managing a Heroic Second Wind and proposing to Yuki.
- Heroic Second Wind: Masaki after nearly passing out from the pain of losing his arm but seeing his final destination and objective is within reach.
- I Meant to Do That: Yuki's excuse for making a knitting mistake.
- Irony: Had Masaki answered the wolf's question when asked, Yuki would have heard it.
- Rule of Three: Mizuho's New Years toast is interrupted three times. By the 3rd time, she's fully expecting it.
- Shown Their Work: Why Masaki sacrifices his right arm rather than his left, despite being right-handed. With the left arm closer to the heart and major arteries, the chances of bleeding to death would have been much higher.
- Squee: All of Shiro, Mikami, and Manaka every time they play back the recording of Masaki telling one of the wolves he won't answer whether or not he loves Yuki until he can say it where she can hear his answer.
- The Stinger: The Book Ends scene.
- Time Skip: A small one to jump from New Years to Graduation Day, then about 15 years pass from the scene before the credits until The Stinger.
- Unto Us a Son and Daughter Are Born: Masaki's child with Shiro is a girl, and a boy with Yuki.
- What the Hell, Hero?: One last one to Kouhei from Moyori, ending with her dumping him after she realizes he loves Sanae and not her.
