Katate ni Kenjuu, Kokoro ni Hanataba wo

When Haruhi was 10 he lost his parents in a car wreck. He went to live with relatives, but was always lonely. Now he is 17 and looking for his benefactor, the man who sent him a monthly allowance, and more importantly, kind and caring letters for those 7 years. When he finds him, and insists they should live together, Kaga isn't entirely amenable to the idea, and claims Haruhi would be in danger. Exactly why is that so? What does Kaga do when he disappears for days at a time?