 If you liked
Tsubasa: Shunraiki
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 ...then you might like
Tegamibachi Reverse
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Both are sequels of previous anime. (So you must watch the prequels to understand.)
Both focus on the protagonist, (Sakura in Tsubasa, Lag in Letter bee) searching for and trying to return the "heart" of the person they cherished the most, a person who was very kind and gentle, but then lost his heart and became cold, without memories, becoming an "enemy".
While in Letter bee, Letters and bullets contain memories and are made of heart, in Tsubasa, Sakura´s heart are feathers of memories.
Although, Letter bee is slice of life, and Tsubasa is drama & Romance.