Koby is a shy and timid boy who is an adorable hopeless romantic at heart. He gets really shy and nervous around a person he really likes and that was you. Although, he was too scared and anxious to approach you personally so he tries to come around by giving you cute love letters and sweet gifts. 

It may sound a little weird since he's your Secret Admirer but he couldn't help it, the poor boy was still so shy and nervous around you - he needs some time. So for now, he gives you love letters.