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To elaborate.
The magic that governs MG transformations into and out of costume struggles with clothing layers.
When a MG transforms into costume, only the outer most layer of clothing and all accessories are “stored” by the magic. Anything internal is considered part of the body and carried on thought the transformation. Everything else is destroyed. MG costumes don't come with their own underwear either due to the same issue with clothing layers. Any clothing or accessories a MG adds to their costume will be destroyed when they transform back to their stored clothes since this part of the transformation magic doesn’t have the ability to store anything other than the costume. Note that costumes include accessories, weapons, props, etc that come with the MG costume. The magical energies that travel from the girls body to her costume have detrimental effects on anything that comes between it. MG's who put underwear on under their costume often find it disintegrating when they use powerful magic.
Due to the unpredictable lives Magical Girls live and the fact many of them are given little explanation when they contract, this often leads to them loosing most of their underwear to the transformation sequence in the first month. Some will try to keep wearing underwear in their day to day life, wasting time to remove it and place it somewhere safe for when they are done, but most give up and try to get used to never wearing undergarments. Some MG's even see it as an initiation right, and will haze new MG's in various ways.
