Walkers are a special type of mob visible only when in soul form. They are one of the dangers present after a player dies, along with having unbound items stolen by criminals. If a soul is caught by one of these grim reaper-like creatures, it'll return to wherever its body died and reduce a character's chance of resurrection by 10%, remove one of the glowing blue orbs on the soul's upper-left corner of the screen, and drop 10% of a random Soul Material. There is a chance the dropped soul material can be picked back up. If a player is caught by a Walker ten times - no orbs are left - that soul is Lost, even if was not in Ash status.
Presuming a death wish is not in order, there are a couple of tricks to escaping walkers. First comes the knowledge that the numbers and respawn rates for these guys just aren't enough to turn a player Lost unless he goes AFK for a while near a spawn point. Do watch out though - whenever you leave the game on and go do something else, try do it with your character alive, as you can always have items stolen or get caught by Walkers (not to mention you turn to ash if you remain dead for over 12 hours [ingame?]).
Walkers walk slower than a character dashes (which you should always be doing in soul form, since you have no OD bar), so outrunning them is not impossible. They possess a charge attack though - as in, they literally run at you in a straight line faster than you can dash, so watch out. Duck around corners or move perpendicular to the Walker if you see him readying a charge. They are also very easy to sneak by: simply walk a couple feet behind their back and they'll hardly ever notice you. Also keep in mind, that even if you die in front of walker - you have around 3-5 seconds to run without agro him and he will not catch you if you stand near resurrection statue.
In a last case scenario, Walkers de-spawn for a while whenever they catch a soul. So if your resurrection chance is already low or if you're too lazy to make another soul run, try to toss a teammate or random player to the wolves. People tend to resent it when you set the grim reaper on their souls, however, so keep that in mind.