Midgard no Shugosha: Re-Birth of Norse Mythology

In this story of Norse mythology, humanity's world of Midgard is caught up in a war between two of two powerful divine races, the Jotunn and the Aesir. Humans can't stand against them, due to the godly 'Megin' power of the divine races. The Jotunn see conquering Midgard as the first step toward attacking the home of the Aesir, and they slaughter many humans along the way. The Aesir step in to rid Midgard of the Jotunn and protect the defenseless humans. One of these Aesir, Thor, made his home base in a certain grateful human town.

In this town lives a young boy named Soah, who lost his parents in a Jotunn attack. He idolizes Thor and dreams of one day becoming strong enough to fight Jotunn and protect people. Thor enjoys Soah's childish enthusiasm, and the two of them share a warm friendship. One day, a Jotunn invading the town uses Soah as a human shield. Thor protects the town, but at the cost of the life of his young friend. Unwilling to let the strong-hearted boy die, Thor bestows his Megin upon Soah and vanishes. After much training, the now-powerful Soah begins to realize his dream, fighting against the Jotunn in Thor's place!

(Source: MangaHelpers)