Are there any items I should be looking out for before I lose the chance to get it? I've read about how there are no missables on this game because everything is available again at NG+, but what about the first game iteration?
I haven't played much yet, cleared the first boss in world 1 (Boletarian Palace), and I'm playing world 2 (Stonefang Tunnel) now. There's a part of world 2 where there's a fireball throwing enemy behind a bridge that collapses if you try to walk over it. After the bridge collapsed, even after I 'reloaded' the map by going back to the nexus, it is still collapsed. There's an item right beside that enemy that I can never reach because of this.
Best Answer
There a few missable items in Demon's Souls.
The first, as pointed out by Jim Bolia, is that worlds have different tendencies, white and black. This determines the difficulty of enemies, souls dropped, et cetera. At Pure White World or Pure Black world tendency, however, the maps themselves sometimes change. New paths may open up, or an additional (unique) enemy may spawn. World Tendency is effected by a variety of factors, such as beating a boss, dying, defeating certain enemies, et cetera.
See below for more details on what you find at each, and how the system works. Effects of:
Changing it:
Source: http://demonssouls.wikidot.com/world-tendency
Next, there are also the Mephistopheles Quests and Yurt, The Silent Chief. The Mephistopheles Quests appear if you kill Yurt, and have Black Character tendency. She will offer you assassination quests to kill many of the NPCs in the Nexus, and rewards you for doing so. Of them, the only unique item is the Foe's Ring. It should be noted that the NPCs will not respawn if you kill them. So doing so is not a good idea before the final boss.
Yurt is related to the above, but slightly differnt. You find him locked up in 3-2, and upon freeing him he will begin killing NPCs (the ones that Mephistopheles wanted you to kill, as well as a few more).To unlock him you need to take an alternate path through 3-2, so it is very possible to miss him. Killing him will grant you the Gloom Set.
There is also the Demonbrandt obtained by fighting Old King Doran. He is located in World 1-1, and you need the Demonbrandt to later obtain the Northen Regelia.
On a final note, the game doesn't force any items on you, and so it's possible to miss more then this on your way through, but for most items you can normally still get them if you just revisit the level.