Dr. Horizon

Dr. Horizon was at first thought to be the person Hilo ran away to, but it was later found out it was Dr. Bloodmoon. Dr. Horizon is Razorwark's human form.

Story
Dr. Horizon first appeared in Book 1 in Hilo's first memory. He appeared in multiple memories after that, and was actually shown at the end of Book 4, Being secretly Razorwark.

Hilo and Izzy greeted him, but then he fell down unconscious. Izzy said it would take a few weeks for him to heal. He woke up at the end, then transforming into Razorwark.