I'm just curious why Misaka isn't putting on the normal panties that's next to Lancis. Why go through the trouble of string panties? Especially since it looks like she's in a hurry with three people coming down the alley.
They're actually changing into bathing suits while trying to avoid security.
So that would be a side-tie bikini-bottom, then? And it must be a bit tricky tying the first strings, before the second ones helps holding it into place.