In Season 1, Episode 11, when Yozora returns to school, her hair is insanely shorter than usual, so much so she looks a bit like a boy, and this is when it hits Kodaka, Yozora is his childhood friend from 10 years go. So what does he do? he stands up in the middle of class and calls Yozora by her nickname "Sora" she turns around tears in her eyes and smiling and she replies "It's been a long time, Taka." (Kodaka's nickname given to him by Yozora/"Sora")
Maria getting Kate to drive her to Kodaka's house so she can give a present to Kobato. What's especially cute about this is that Maria (after a little prodding) straight up admits that that's why she's there, as opposed to a Suspiciously Specific Denial like the other Tsunderes in the Neighbor's Club.
Maria: Here, I decided to be nice and get you a present, so you better like it. Birthdays are kind of important, even for a poopy vampire like you.
And Kobato's response, after being told to say thank you by her older brother. She barely manages, but she tries.