@LoupDeGarou and @stilgar could be a mishmash of religious characters, I've always pictured Zeus with a plain lightning bolt, or throwing javelin that forks, this gramps had a pretty clear trident, so he's either a cross between Zeus and Poseidon, or someone decided that an isekai Odin would be a Lightning God if he just added an N to his name....
@CountryMage My first thought was Odin, but he doesn't use thunder if I remember it correctly. That's why I went with Thor. But the option with the highest probability of being right is that the grandpa ghost is a mix of multiple gods.
@TNT261
To be fair, humanity does treat him like trash, and his only reason for going on the journey in the first place was to send money back to his grandparents.