Here we have Nastasia/Nautilia the Trident Of Marrow (left), Laura/Lora the Saber Of Blood (middle) and Izsha the Zerg Advisor enjoying peaceful time on Summer Island, that's why they wear bikinis/swimwears here. Another reasons? Nastasia because she grew up with humanity since she was adopted by Stukov himself, Laura so she remembers her true self and Izsha in order to fit between non-Zerg locals somehow. Also, this is how Izsha could look if she could go all by herself from time to time.

(Izsha belongs to Blizzard)