public OnPlayerSpawn(playerid)
{
     if(IsPlayerNPC(playerid))
     {
          new npc_name[24];
          GetPlayerName(playerid,npc_name,sizeof(npc_name));
          if(!strcmp(npc_name,"Name_Of_Your_NPC",false))
          {
               SetPlayerSkin(playerid, 264); // your skin choice for NPC
          }
          return 1;
     }
//rest of callback here