One (Levi)

Levi is determined to find a mermaid who can heal his leg, as mermaid's tears are rumored to heal any wound.
When Eren gets captured by Levi and his crew, he refuses to give Levi a tear until he earns it.
Levi tries hard to earn Eren's tear, and the two grow closer.

Liked this story? The sequel called "Tails" is also a finished story.