In Free! marine biology au the mermaid Gou is as interested in the marine biologist Seijuro as he is in her.
Biologist Seijuro approaches the merfolk pretty freely and gently, allowing them to get familiar with him. He swims with them, studies them from the close range and often sits drawing them. Seijuro always has a camera with him but he prefers handwritten notes and drawn studies.
Though Seijuro works with any merfolk, he’s the most familiar with the shark types, tutoring also Nitori. He’s bonded well with a mermaid he named Gou. Gou’s brother Rin - also named by Seijuro - still refuses to accept Seijuro fully, but sometimes comes to disturb his work. Like dragging his sister back into the water away from Seijuro’s sight.
dnaegi suggested that Seijuro could be one of the biologists, who approaches merfolk by swimming in the same waters with them, befriending with Gou - which annoys Rin. I thought that was brilliant idea! He definitely would go to swim with merfolk and spend hours and hours studying them with his enthusiastic nature.