Natsume's Book of Friends (Natsume Yuujinchou in romanized Japanese) by artist and author
Yuki Midorikawa is my all-time favorite manga and anime series. I've been reading, watching, and collecting it for years so I thought it was high time I showed my love for it here on the blog!
The way I describe the story in a nutshell is "slice-of-life with yokai",
yokai being the word in Japan for supernatural beings/creatures (to put it very simply). Natsume, the main character, was born with the ability to see yokai and interact with them. The story focuses on the impact this has had on his life, in both the human world and world of yokai, and how, through various positive and negative experiences, he matures and begins to reconcile the two, learning that they are sometimes not all that different. Perhaps reticent by nature and certainly taciturn for preservation, appearing aloof and slightly frail, Natsume proves time and again to be passionate, loyal, brave, and possess a deep well of strength both physically and emotionally. Nyanko-sensei, a large and powerful yokai who chooses to be seen in the form of a plump cat, similar to that of a
maneki-neko (or luck cat figure), is always by Natsume's side (unless he's off drinking with his pals or pilfering sweets). He acts as Natsume's mentor and bodyguard, and, though he tries to act tough and deny any real attachment to the boy outside of practicality and personal gain, both the readers and Natsume know he's a softy at heart, willing to protect Natsume even in the face of danger to himself.