To me Killingham looks like the rookie of the Holy Knights. Look how nervous he is on his first day at work and he has never been Elbaf before. He is wearing a bubble like the fodder CD. His mother must have told him to wear one so he doesn't breathe dirty commoner air
Gillingham having a helmet and appearing in Hybrid form interests me. He was Teleported while changing, there was no reason for him to be in Hybrid.
What if Gillingham has Sapphire Scales, or some other disease, and can't actually breathe natural air, and hides his human form, because it's diseased?
Oda is generally very intentional with design choices.