The feminine counterpart is girlvoid.
History
The terms boyvoid and girlvoid and their flags were created by @thebeardoesart on Instagram[1] on September 19, 2021 to relate to how the user feels about his gender, which the label voidboy didn't correlate with.
Flag
The white stripe represents being partial/somewhat, the green stripe represents gender neutrality, the light blue stripe represents being male/male-aligned, the dark blue stripe represents masculine aligned neutrality, and the black stripe represents being gendervoid.