Koi Kaze/Tear Jerker

Koi Kaze isn't the happiest manga out there to start with, but there are still some extra tearful moments

Koshiro: "I don't plan on trying to contact you again. This is the last time we'll be together like this."

Shortly after, when Nanoka begs Koshiro for a kiss before he leaves forever:

Nanoka: "Only if you give me a kiss ... I'll promise never to see you again"

She closes her eyes and lifts her tear-stained face to Koshiro and he ... kisses her on the forehead, says "Goodbye" and starts to walk away. She yells "Unfair!" to which he responds "Just appreciate it." Before he walks off, she wipes her eyes, gets his attention one last time and gives him a happy smile and waves goodbye, then turns and runs home with tears streaming down her face.

Koshiro and his co-worker wind up in a store that sells wedding dresses with Nanoka tagging along. Nanoka is persuaded by the oblivious co-worker and the shop-keeper to try one on. Koshiro is stunned by how beautiful she is, while also knowing that this is the only time in her life she will get to wear one if they keep their relationship. Then, they get their picture taken next to each other, as if they were bride and groom. Nanoka keeps the picture in her journal and looks at it often.
Koshiro makes up his mind and tells Nanoka:

Koshiro: "I ... don't need anything else ... if you don't need anything else either."

Which is his way of finally telling the rest of the world to go to hell about their relationship

Nanoka: "If I could be reborn, and not be your sister ... so then ... I could be your bride."

Most of the ending seems to imply that their relationship is doomed. The manga is slightly more upbeat, but during the last couple of chapters Koshiro admits that one of his childhood dreams was to get married and have children, and says that he will never have that dream come true in exchange for what he really wants.