Eiden High: doodles (Patreon)

I've been thinking about mechanical aspect for Eiden for a while now, and it might be an interesting idea to give him prosthetic lungs and heart due to some severe injury he had received in the past from, probably, an unhappy co-worker, a client or, which will be even more tragic, from a loved one. This way it might explain his, almost always, emotionally detached behavior towards people and serious tone despite the situation, and leaning more towards annoyance and irritation as defense mechanism. 

From what we have now, his brother, Everen, and Rah are the closest people to Eiden. It can be explained as Everen is a spring of undying will and optimism, and also occasional kicks in the butt, and Rah as sort-of oblivious emotional support to Eiden with all of his quirks (which Eiden would always deny that he likes them), and a genius mechanic who have replaced human technology with Arrach-Ray's, pretty much saving Eiden from the pain and failures that the first ones caused.