redappendix's Things

i remember reading this manga but i never added it and it was about some guy who i believe was held in questioning for some murder he never committed and he helped solve it and on like the last chapter i read a few months ago he solved some weird letter things that the girl he sat next to on a train kept receiving from her birth mother (?) because there were always a bunch of symbols and drawings on it and he deciphered it using the syllables of the words

(ps the manga was like black and white)

pls help me find it i would be so grateful if you could help me