Scarlet Pensar is a pyrokinetic, so people would think her weakness is water. However, she has learnt to adapt to water, mainly due to her magic usage. So water is not a big problem to Scarlet, and she usually wears diving equipment when underwater, and uses her magic to infinately fill the oxygen-tank.
However, this does not mean Scarlet has no weaknesses. Her most notable weakness, due to it being her biggest, is tar. Not only is it sticky and hard to escape from, but it disallows her to use her pyrokinesis due to its high flammablilty. Not only that, but it also makes movement very hard for her if she gets covered in it, which means she cannot use her spectre. Some of her enemies know this weakness and use it against her. She cannot escape by natural means and needs help in order to get free. This is probably why Scarlet has never liked tar at all...
(she may have more weaknesses, so stay tuned on this page if you wanna know how to defeat or weaken Scarlet, or just find the page interesting)