Milky Holmes are

- Suzuko Mimori as Sherlock Shellinford,

- Sora Tokui as Nero Yuzurizaki,

- Mikoi Sasaki as Hercules Barton, and

- Izumi Kitta as Cordelia Glauca.

They are a seiyuu unit from/for the anime Tantei Opera Milky Holmes. When performing a live, each member wears a certain cosplay to look like her 2D counterpart from the anime.

Company profile: https://hibiki-cast.jp/hibiki_f/milky-holmes/