Tegami Bachi Vol. 7 (manga)

Blue Notes Blues

Lag has discovered that his long-lost hero Gauche Suede lost all of his heart, became tangled up in an anti-government organization called "Reverse," and is now known as "Marauder Noir." Noir’s trail leads Lag and Niche to the icy northern town Blue Notes Blues. It just so happens Niche was born here. Lag and Niche uncover her mysterious origins: a sacred underground lake...where her brethren still dwell.