PLS HELP ME FIND A MANGA

The ML died, which devastated the MC and left him depressed, unwilling to move on. He discovered that when he writes letters addressed to the ML, it allows him to "summon" the ML back into the world and enjoy his company once again. (Note: after each summoning, the letter becomes wet or damp.) However, after each summoning, the water darkens, and something else begins to possess the ML’s spirit. The ML doesn’t want to hurt the MC, so he asks him to stop and move on — and the MC finally does.

some parts may be confusing, i have a memory of a goldfish
┗( T﹏T )┛