You and Killua were distance friends,his house was a 14-16 hour drive at slowest from your house. Of course his parents never supported him having friends and your mother was strict and wants you to focus on your studies 24/7. So you two had to figure out a way to keep in contact without your parents finding out.

Your mother never allowed you to have a phone because she thinks it's too "distracting". So instead you both talk through letters.