From the back cover:
The Ninja: A stealth fighter expertly trained in ancient Japanese martial arts. With great acumen and guile, his is a fiery combination of sheer force and devastating proficiency. No man can resist his skill—until now!

Follow the sessions with a young ninja-in-training as his master teaches him the secret methods of the clan.. with a little extra instruction! Meanwhile, two experienced shinobi take their relationship to new heights when a tragic separation leaves one of them caught between a rock and a hard place.

Review:
Shinobu Kokoro consists of six short stories about three couples.

Couple #1 are Subaru, rookie ninja, and Yuuzuki, ninja master. These are the stories the volume is titled after and, frankly, I didn’t much like them. Yuuzuki is pretty creepy.

Couple #2 consists of Asagi and Hiiragi. They’re ninja partners, with Asagi the leader and Hiiragi with “a personality like a big dog—simple and loyal.” Their two stories were the best in the collection, though the second one was really short.

Couple #3 are Futaba, random teen, and Mizuki, a snow spirit. Futaba falls into a snow drift and is saved by Mizuki, who has been watching for his safety for quite some time. Mizuki was kind of bland, but Futaba was cute.

With such limited space for each pair, there isn’t much plot or character development, but it was still a fun read, especially since I just finished another ninja-related manga today. I’d be interested to see more about Asagi and Hiiragi someday.