According to Joule's bios from Gunvolt 1 she was Sumeragi's first artificially created adept. That is to say, a designer child implanted with a septima before birth. It's implied heavilly by Mytyl's poor health and subsequent recovery that a septima is a fundamental part of an adept's soul. So its possible Joule was essentially a fragment of Mytyl by nature upon birth. Joule's body fading...