The king, Luci's papa.
The man is ridiculously huge, almost too big for his own home though he's got minions constantly making his palace bigger. Satan is of course, doomed to remaining in Hell. He's unable to leave and frankly, he's very tired of being there. The only thing he seems to value and put above anything else is Luci. So letting his son leave and traverse the human realm makes him feel very unsettled, but what helps his nerves is giving Luci the task of gathering shards of this precious gem that could very well be his key out of Hell.
Satan doesn't seem to question why Luci is able to actually leave Hell though. He shouldn't be able to but with Luci gathering those shards, it probably shouldn't matter much at all...!