K2 中文 teaches 600 core Chinese characters and words from HSK 1 to 3, with pinyin, meanings and writing practice. Each card asks for the meaning, the pronunciation or the character itself.
Pinyin is available as a hint when you need it — but a correct answer with the hint showing cools the card more gently than one without, so you are nudged off the crutch.
Every card has a temperature. Get it right and its chance of coming back halves; miss it and it doubles. K2 picks the next card at random, weighted by that chance, so a session keeps returning to whatever you are getting wrong and lets what you know fade into the background.
When the cards you are working on have cooled, a new batch joins them, so you always study at the edge of what you know. There are no review dates and no daily queue to fall behind on. Every drill is free and works without an account, and everyone gets 20 free AI uses a day; past them, signed-in AI practice runs on your Bike4Mind credits.