How I think it is: is that you gotta be naturally skinny, super skinny weight with normalfag biceps, triceps, etc. Arms must stay super.
I'm 5'10 109.5 lbs, always around there.
Do bodyweight abdominal exercises, NEVER lift/gym, never ever you'll just fuck up your silhouette. Train with a sword since a young age like Kenshin so you'll develop just the areas needed for a fast, dexterous usage of a 1 lb 12.4 oz Katana.
which develops shoulders & back very little so they stay flexible but resistant to the fatigue of swinging a sword for too long.
You need to stay in Japanese Visual Kei weight.