@Mewples he can assume dragon form, it's just that he's gotten very jealous of all the great food Rimuru was eating, so he decided to assume a humanoid form even though his dragon form is better at combat (read: unyielding firepower).
@Thifa the reasoning was a bit better in the WN, the Munos had an obsession with heros and therefore decided to name satou after one, hence the Pendragon surname. I might have gotten some details wrong though, it's been a while since I read the thing.
This also adds a bit more to Sauro's motivation, he thinks that he's the reason Arsenio wasn't "protected by the lord". He thinks that it's his fault that Arsenio is, in the eye's of their religion, a corpse running around on the whim of some malevolent beast.
@lenovoaxioo to explain "holy demonic" means the resistance applies to stuff based off of darkness i.e. demon/monster stuff and light based stuff i.e. that disintegration thing Hinata used against the clone and the barriers used to weaken tempest.