So, here's the Loki theory I was cooking up with Reloaded (well, he listened to me rant, LOL):
I think Loki has a Fruit that allows Shapeshifting, and it's the Fruit he got from killing his Dad. In mythology, Loki could shapeshift and change sizes.
This will allow Loki to fit on normal size ships.
Harald looks like Loki, because Harald was in love with a Human woman. We know from the New World that Oda likes size and age gaps, with Neptune/Otohime, Kyros/Scarlett, Oden/Toki, Gyaban/Ripley...
Harald used his Fruit to turn into a smaller man, and could Shapeshift to remove his horns, too. He wanted to understand humans, and used his Fruit to travel to human societies. Harald fell in love with a Human woman, but, concealed his race, and pretended to be a Human. She loved Harald, but hated Giants. So, Harald felt guilt. Then, a tragic miracle:
Pregnancy. Harald's Human lover was having a child. The problem? It had Harald's Oni Genetics, and, because she was a Human, Harald's Human lover, like Portgas D. Rogue, died giving birth to her child; Kai D. Onisomething.
Harald's heart was broken. The horns on his head, had led to the horns on his son's head piercing his lover's stomach during birth, disgusted him. Harald ripped off his horns, and decried the violence of the Giant race even further.
Knowing his son would be rejected/hunted by Elbaph for being a human/Giant hybrid made by the King of Elbaph, Harald had him sent away. However, something went wrong, and Kaido ended up in another Kingdom.
Loki found out about this/Harald's deal with the WG to make the Giant's Mercenaries, and killed Harald over it.
Harald was planning to betray the WG, though, which is why Sommers is pissed; Harald trusted Loki and Hajrudin to overcome their differences and lead the Giants, even if he died.
There's probably some prophecy shit in here, because Norse Mythology is full of prophecies.
So, Loki should have Harald's Shapeshifting Fruit, which will allow him to become human sized, and sail on a ship
Loki for Straw Hats? Or Blackbeard? KIDD???
I think Shiki is Mosa, and will free Loki when Luffy leaves.