Teru Teru x Shonen (2001)

Blurb:
Six years ago, young Shinobu left her rural village to go to school in the big city. Before she left, she told her friend Saizou to grow up strong so he could someday come and protect her. Now, at age sixteen, shes fairly well adjusted to life at the Tokyo Private Imperial Academy. But when a male exchange student comes to the school, it turns out to be none other than Saizou! Although his appearance hasnt changed much, he is now a skilled ninja, and his arrival couldnt be more timely: someone is out to get Shinobu, and no one knows who or why. Saizou wants to be more than a protector to his old friendbut he’s got a rival for her affections.

Source: www.anisearch.com/manga/52