Those entities are the playable characters of the game.

At every level change I check (after loading the level) which of those characters are located in the current level and get their position.

Then I use ent_create to put those characters at the saved position inside this level.

That's the code I use:

Code:
function check_character(CHARACTER* c_player, ENTITY* ent_player) {				
	if (ent_player != NULL) {				
		return; // returning from check_character. player was already set.
	}
	if (str_cmp(c_player.isInRoom,level_wmb) == 1)  {								
		c_player.isInCurrentRoom = 1;
		c_player.head = NULL;
		c_player.entity = NULL;
		ent_player = ent_create (c_player.mdl, c_player.position, player_movement);		
		ent_player.CHARACTER_POINTER = c_player;		
		ent_player.emask |= ENABLE_IMPACT;
  	ent_player.event = push_event;  	  	  	  	  	
		VECTOR tmp;
		vec_for_vertex(tmp, ent_player, c_player.headVertex);		
		c_player.head = ent_create (c_player.headMdl, tmp, NULL);		
		ent_player.HEAD_POINTER = c_player.head;				
		ent_player.pan = c_player->rotation.pan;
		ent_player.tilt = c_player->rotation.tilt;
		ent_player.roll = c_player->rotation.roll;
		ent_player.link.name = c_player.displayName;
		ent_player.string1 = _chr(c_player.displayName);		
		ent_player.scale_x = c_player->scale.x;
		ent_player.scale_y = c_player->scale.y;
		ent_player.scale_z = c_player->scale.z;
		c_player->head.pan = c_player->rotation.pan;
		c_player->head.tilt = c_player->rotation.tilt;
		c_player->head.roll = c_player->rotation.roll;		
		c_player->head.scale_x = c_player->scale.x;
		c_player->head.scale_y = c_player->scale.y;
		c_player->head.scale_z = c_player->scale.z;		
		c_player.entity = ent_player;				
		wait(1);
		ent_player.flags = NARROW | FAT | FLAG2 | FLAG7 | FLAG8 | POLYGON | SHADOW | CAST;
		c_player.head.flags = PASSABLE | SHADOW | CAST;		
		c_setminmax(ent_player);	
		c_updatehull(ent_player,2);			
	}	else {				
		c_player.entity = NULL;
		c_player.isInCurrentRoom = 0;
	}	
}


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