Relatively early in the forest of the lost giants you climb up a ladder and there sits a white knight beneath a tree. He looks like an NPC, he does not attack or react in any way, but there is also no talk-prompt for me to use. What's up with this guy?
He is a Heide knight. There are several of them in the game and he won't attack you until you attack him. He's not a regular NPC and doesn't talk, but he is much stronger than the other enemies and if you kill him, you get a good weapon. As far as I know, they are just normal enemies and offer no other possibility of interaction but killing them.
There are 3 in the game. All of them are non-hostile, unless provoked. Especially the one in Forest of Fallen Giants can prove a huge challenge to new players, because of its wide range and immense damage.
In normal mode, they drop the weapons Heide Knight Sword, Heide Spear and Heide Lance.