Ningyo no Fue

When he was little, Tado lost his parents in a drowning accident, and has lived with Twi, a baby mermaid he met on the beach. Though they were different species, the two were as close as real siblings, but as Twi grew as a mermaid, life as a human became more difficult. And then the time came for them to part ways. A touching tale of parting between a human brother and a mermaid sister. (Source: Hakusensha, translated)