As for what they were exactly, Wraith-Walkers were considered a unique and rare breed of Undead entities. Though unlike other Undead creatures like Zombies and Vampires, who increase their population through an infectious bite, Wraith-Walkers are biologically born this way, and they don't gain access to their powers until they turn the age of 13 to 14, where they would have to partake in a ritual to fuse a restless spirit, a Wraith, to their very souls.
In turn, this gives them access to a host of special abilities and techniques including the ability to effortlessly shift in between the physical world and the "Wraith World", which is a mirrored version of our world where these restless spirits call home, hence the name. However, unlike other Undead creatures, Wraith-Walkers are one of the only Undead that looks very much alive, instead of having the appearance of a rotten and decaying corpse, allowing them to blend into human society much easier than other Undead.
Though unlike Zombies and Vampires, who have one or more weaknesses that can spell out their demise, Wraith-Walkers are extremely hard to kill, due to having two souls housed inside their bodies instead of a single soul, which means that even if they are physically killed in combat, a Wraith-Walker would eventually resurrect, which would take from around several hours, days, even a week or two....depending on the severity of the inflicted wounds, of course.
Of course, Dalton was born with this bloodline in his veins, as he then heard a voice rang out to him and said, "So...you're still thinking of *Her*, aren't you?" Looking to his left, Dalton found out that it was none other than....