Derek, also called Iron Maiden by some fans, is a chaser that stays in the Silent Hill? area. If you get too close to him his cloak opens revealing a mouth or ribcage like area. If you try to hit him with the bat effect it does nothing, however when you walk away over time he starts chasing you.
Having the highest possible speed settings, Derek can catch you within seconds even if you have the speed effect equipped. The only way to avoid him is either getting to an exit by luck or being lucky enough to simply be warped back to the beginning of the area when he catches you.