What are Seiko and Mimi wearing?

In the Anime I noticed that Seiko is wearing maybe a waist trainer(?) which could be part of the reason why she appears so young or maybe a spiritual reason. But I was also wondering if anyone happened to know if they’re wearing compressions socks and sleeves? Right now I’m only caught up on the anime so maybe there could be an explanation for their attire later on but I’m just curious if anyone knows if they wear it or if there’s a reason for it. Thank you!