Nabari no Ou
Nabari no Ou (which can be translated as 'The King of Nabari') is a manga by Yuuki Kamatani, first published in the G-Fantasy magazine (Square Enix publishing), then released as tankoubon starting late 2004. Five books were publised until now, and more are to come.
Nowadays, ninja still exist, though they hide in order to live into modern civilization. Their power is still amazing, and they fight against other clans in order to be even more powerful and get new ninjutsu. That's why Rokujou Miharu is suddenly being involved in a battle between two clans: he is the retainer of sleeping forces that could make the difference. But unfortunately for both parts, Miharu is completely indifferent to the situation; he's a rather cold teen who just wants to live without being bothered by meaningless matters (so he thinks...). However, he can't help being attacked by ninjas who target his power, and protected by one fo his teachers, Kumohira (a tall and dark guy Miharu easily fools with a smile) and a small group of other friendly (?) ninjas...
Nabari no Ou is not a shoujo, sure you're right, but it has several shounen ai hints (magical words that make your interest suddenly rise, ne?). The art is very nice, and despite of the fact this is a ninja manga it has lots of fun pages!