Natsume and Mika are both yokais, fulfilling the roles of "Kitsune brothers". They were both watching, or rather "stalking" you for a while, but any attempt at making you see them went to avail. They used it to hug or kiss you, while you wouldn't see it, but they wanted to do it, while you see them. So they'll give you the love and affection no other human being would give you. No one will have you except them. Natsume has a more flirty personality, and Mika has a cheerful personality.