More newbie questions time!
I should preface that I'm playing with the Fextralife wiki walkthrough so I don't miss anything. The story and exploration isn't something I care TOO much about...I'm more enjoying the gameplay experience.
1. I just realized that I have 3 covenant items, but I don't know what they're for.
2. I have no idea how online works. I've never been invaded nor have I ever tried to invade anyone or go into co-op. I don't love the idea of randomly having someone spawn into my game and killing me (thus making me lose all my souls) when I'm trying to move forward in the game. But I feel like someone could make the online side of things make more sense to me to see if it's something I should start doing.
3. I keep reading about killing certain NPCs. Why would I do this? More often than not, I'm reading that I should just go on a murder spree before I start NG+.
4. Will there be enough mats to upgrade any weapon I want, or will I have to be selective on this?
5. Are there any truly missable items up to the final boss, or will I be able to go back and get shit that I forget/leave behind/don't know about?
1. You can either read the item descriptions or just try them out.
2. Not sure what you're asking exactly, but if you're kindled/ember you can be invaded if your boss in the area is alive. The invader uses a Red Eye Orb to invade, and depending on which covenant they're in, they'll receive a reward if they kill a host. When the invasion is "over", the host will be set on a cooldown so they'll have no more invasions for a while. This does not apply for two covenant areas though - where you'll be invaded more often because you're trespassing their territory. There are also mad phantoms which you can manually summon (via red/pink signs on the floor) if you want to PvP. White signs are phantoms who wants to help, and will disappear if they die or if you defeat a boss.
If you join Way of Blue you have a chance of summoning a Blue Sentinel, who will help you if you're invaded.
3. It depends on the NPC. Sometimes they will go hostile for some reason, and sometimes killing them will be part of a quest. But for the most part people just want to kill them fpr their gear or something. Usually a thing you try before going to NG+.
4. There are definitely enough materials in the game to upgrade several weapons to +5/+10, it just depends on how much you manage to find. I wouldn't worry too much about it, just upgrade a weapon you like to use and make sure to try a few of the ones you find.
5. You can go back and get the items 95% of the time, yes. The things you can miss are mostly related to NPC quests.