Shinobu Kokoro: Hidden Heart (2004)

Blurb:
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 sizzling sessions with a young, innocent ninja-in-training as his master teaches him the secret methods of the clan… with a little hands-on instruction! Meanwhile, two experienced shinobi take their relationship to new heights when a tragic separation leaves one of them caught between a rock and hard place. Filled with over-the-top “action,” boys in tight uniforms, and one very lascivious snow spirit, this romantic fantasy collection is one part passion, one part affection, and all heart. Shinobu Kokoro: Protecting the clan with some man-on-man!

Source: www.anisearch.com/manga/1204