Ok, now I have some questions to people familiar with the game:
How does multiplayer work in this? Is it Souls-like? Can you play the entire game in coop?
It has elements that are similar to Souls games such as the amirita being like souls and the shrines being like bonfires, some of the levels feel like the interconnected areas in Demon's Souls and that is about it. You can play the game in a Souls like fashion if you wish as the combat mechanics are the most comprehensive I have come across but that is just one way of playing. It is just as much inspired by games like Warframe, Diablo, Destiny and pretty much every successful ARPG in the last decade or so, many of the mechanics in Nioh are far closer to what you can find in those games than the Souls inspired mechanics.
If you want to play it like a character action game you can, if you want to play it like a Souls game you can, if you want to go min/max looting where you destroy everything in a few hits you can. Within that there is then a huge amount of variety by using different skills, items, weapons, ninjitsu and onmyo. The first area feels the most souls-like until you get access to armour sets and skills then its up to you.
Not entirely sure how the coop multiplayer works since it has been updated. Originally you had to have beat areas to do it but I think that might have changed. I'm pretty sure you can play the majority of the game in coop going by a few comments in other threads. There is PVP but it was very unbalanced when I last played it. There is the revenant system where you can get higher level gear by fighting players that have died. There is an online guild type thing that gives you extra perks as well.