all your fave characters are only cis in your headcanon
unless they actually show a baby being born on-screen or on the page, and the doctor actually saying, “It’s a boy!” or “It’s a girl!” (gender assigned at birth)
and then also show the character grown up and actually saying themselves, “I’m a boy!” or “I’m a girl!” (gender self-identification)
then there’s no canon proof that a character is cis. That’s just your headcanon, and tbh it’s a pretty boring one.