This question's so fun omg!! I made them with the intention of them being a couple from the start. I really love shipping and drawing cutesy things, so I knew I wanted to do that.
I definitely wanted them to have a height difference and aesthetic difference, as well as making sure they're both character archetypes I enjoy! The idea of a monster/magical girl randomly appearing in a normal girl's life was so cute to me, I love fish out of water type situations... So yeah, that!