Given the unlikely origin stories of some superheroes, it's somewhat puzzling when they turn out to have relatives who share in their abilities. In the case of Martian Manhunter, he has a cousin called Miss Martian. There's quite a strong family resemblance, but given the fact that they're shapeshifters, we suppose they could choose to look like identical twins if they so desired.
Martian Manhunter's costume (and general appearance, for that matter) are very alien. He borders on hostile in the way he presents himself, so his cousin makes for a significant departure from his approach. Her costume shares many design elements but definitely has a more human touch. It makes it much clearer whose side she's on. She retains the elements of mystery and enigma that work so well for her male cousin, but she makes them more relatable and accessible. To be frank, we find her more interesting as a character.