Does anyone know how the Multiplayer will work?

The only information we have on multiplayer was given out through stretch goal updates during the campaign, everything else is speculation. I'll repost what i know of for you real quick though.

Update 1: $1.25 MILLION UNLOCKED: Local Co-op! Now you'll be able to assault this particular lonely, demon-filled castle with a friend.

Update 14: Asynchronous Online Multiplayer
You've been guessing at this one for a while, so it's time to reveal a few things IGA's hoping to implement!
* "Borrow" a friend's character stats for a certain amount of time (your friend will get a bonus)
* Ghost Data (See friend's saved character movements while fighting a boss)
* Friend's playthroughs affect your game in some way (e.g. a gravestone and an offering might mark the spot where your friend died)
* Plus another surprise that we'll unveil soon...
Of course we're still in the planning phase, so who knows what we'll end up with. As always, though, IGA and the team would love to hear your ideas—how would you build a great asynchronous multiplayer experience?

Update 18: ONLINE CHALLENGE MODE:Compete and cooperate with a friend to earn points in specially designed stages by clearing challenges, killing enemies, collecting items, and more. Rumor has it there there may even be incredibly powerful bosses which drop rare crafting materials...

I believe that is everything but if anyone else knows I'm sure we'll find out soon.

Well, Local Co-op and, as someone said, Online Asynchronous Multiplayer which were both funded goals. Local Co-op is pretty self explanatory but the other could practically be written in French since I have no idea what that even means.

The Rouge-like Dungeons seems to be only mode that I currently know about that could be multiplayer. Then again, there may be modes they are implementing we don't currently know about.

No idea how many players they will support though. 2 is obvious but with 3 playable characters, I'm not actually sure.

I really hope that the local co-op is the same as the online one. It'd be weird if there were two different modes or something like that. :/

This hasn't been confirmed yet but the team has a plan already laid out. The local co-op and online mode will most likely have the same features. 
As far these features being in the full package, this needs to be discussed with our publisher. We'll let you know when the plan is more solid .

Oh Mana, you always know how to make me feel all warm and fuzzy on the inside. ^-^

As someone having rather a lot of difficulty finding good coop games on the PS4 (My friends tend to have different tastes to me either being competitive players or liking different types of games) I'd love to have as many modes as possible be coop both local and online. I dislike it personaly when multiplayer has limited options. Still so far this seems like good news to me, and I'm glad that so far people seem to have similar opinions. 

Of course, full coop in every mode both local and online may be a bit too much to ask and a lot of work. But I can remain hopeful.

Besides, I feel as though with not as many coop games on the PS4 (That I can find anyway) there is a good niche there.

How about "borrow a friend's familiar"? How about that?

Yeah, though I don't imagine death will be quite so common or profoundly important to learn from in Bloodstained. It might just be a neat marker or used for some other purpose. In Dark Souls, terrain, trap, enemy and PVP locations are all identified/informed to the player by the bloodstains and play back the nature of their demise. Such a thing moved over to Bloodstained would not be so useful - they'd likely just be in boss rooms or when someone happened to get stuck somewhere without healing items. There aren't really myriad ways to die, like "oh this person fell in a hole here" - it'd be more just like "oh, my friend Billy couldn't heal anymore here" lol.

I'm interested to see exactly how they use the asynchronous stuff in a more 'vania way. Maybe hints as to very obscure secret areas or puzzles so obtuse you kind of need a push? Maybe enough players/close enough players are using a certain power in a certain area and it has an effect in your game. A lot of these features are basically in Souls games to offset difficulty though, and I don't think that will be completely necessary...maybe good for Nightmare mode though?

Well I see what you mean, I guess I am still hoping for a variable difficulty setting to satisfy hardcore igavania and/or Castlevania fans as well. xD I personally like difficulty a lot. (thus a huge fan of CotM)  To stay on topic, it should work in a slightly different way than in Dark Souls I agree. (to reveal secret rooms that way would be neat idea as well) If taking anything from the Dark Souls asynchrous multiplayer, preset messages which players could drop on the floor for other people to read would fit best in Bloodstained I think. I still like the idea of gravestones (bloodstains in Dark Souls) which gives you some information about a player's death.