200 Years of Loneliness is the 25th overall chapter of Tegami Bachi Manga
Confused and angry, Niche's sister is shot by Lag with his shindan, revealing the time he first met Niche and their friendship. Despite this, Niche's sister believes Niche has forgotten how the humans treated them and attempts to kill her and Lag.
Another flashback shows that both sisters were always together even though they were mistreated by the townspeople, until they were separated when Niche was washed away from her sister. Niche's sister sinks to the bottom of the lake and grows sad, eventually transforming into her adult form. Instead of searching for Niche, she decided to stay in the cave where the Maka resided as she believed that Niche would sense her heritage and make her way back as she did. The vision concludes.
Lag throws the sister back to her senses by saying she was foolish like a human as well. Her heart softened, and therefore, she takes Niche to Tir na Lake to heal her wounds. She also explains to Lag that the gaichuu frozen in the cave are metamorphosing when they shed their last piece of heart; however, the Maka froze them to prevent them from causing chaos to the world.