I Want to End this Love Game

Yukiya and Miku started saying I love you to each other in primary school, and the rule of the game they invented was that whoever lost composure first lost the round. Eleven years later they are still playing. The series picks up at the start of their second year of high school, with the game escalating into territory neither of them is sure is still a joke. Their classmates have opinions. Their old friends have opinions. The two of them have grown into people who can no longer say the phrase aloud without meaning it, and the game's exit clause has not been worked out.