User blog comment:B214/Question and Answers 6/@comment-82.2.190.195-20161106015246/@comment-9595671-20161106032538

Technically, Issei and Ddraig were already a part of each other at birth. Sacred Gear users are essentially born with their Sacred Gears so I think it's accurate to say Issei is the reincarnation of Ddraig's destroyed body. So to a degree, Issei is Ddraig.

When he gave up his arm, since Ddraig' soul is in the sacred gear, I'd say it was like he was letting part of his original form remanifest itself. That's likely why Ddraig needed to use all that energy, he wasn't just restoring Issei, in a sense he was healing, because his host is in a sense himself in another form.

Atleast, that's how I'm seeing it. The main reason behind my thoughts is the fact Ddraig's and Albions past possessors were within their respective sacred gears.