〜っけ
asking about something you once knew but can't recall
「っけ」 is used to check something you "should know but can't quite recall right now."
⚠ Key restriction: it can only be used for things you once knew.
• 名前 何だっけ? ("What was your name again?" I knew it and forgot)
• 名前 何ですか? ("What's your name?" I never knew it)
You cannot use っけ to ask about something you never knew.
So what this sentence implies is: we're so close that I should know what you did, but I can't remember.
(In Episode 10, scene 20, the last line of dialogue in the whole drama is 「何で『ぴょろたん』になったんだっけ?」, a small thing all four of them should know but none can recall.)











