Ordinance #133 series of 2006 from the Traffic Management Code of Marikina City states that motorcyclists must wear a helmet at all times while in transit. Failure to do so will be fined P500.
You may think that you can get away with it for short trips and trvavelling during odd times of the day. But Marikina City traffic officers put up random checkpoints at random times of the day within the city. So rather than take a chance, it's better to abide and wear your helmet.
Besides, it's for your own protection.