Giratina is more of a general stand-in for the underworld then Satan on his own, particularly since nothing about its story shows it has any resentment towards Arceus nor try to spread evil to the world or anything of the sort.
While I agree that Giratina is not evil. I do see it as the pokemon parallel to Lucifer/Satan.
It was created by Arceus (god) as one of the first pokemon. But was banished because it was too violent. And it's the "Renegade" pokemon. Rebelling against god just screams Lucifer to me.
Regarding Tapu Koko's standing as legendary or otherwise: the official site doesn't categorize it alongside Solgaleo and Lunala when you filter for Legendary Pokemon, instead it falls into the Other Pokemon category.
Not to imply that it won't be treated as such in-game, but as of right now it's a regular Pokemon.
Magearna is classified as "other" as well. But it's still a legendary pokemon. But both Magearna and Tapu Koko have larger images on the official site than the other pokemon when you look at the full list. The only others with large images are Solgaleo and lunala. So it's pretty safe to assume Tapu Koko is legendary seeing as it is considered a guardian deity, gets it's own special section of the trailer, story, signature move and unique ability.