Natasha’s defining traits, I think, are:
- She’s Russian
- She’s a spy— the genre aspect here isn’t always emphasized, but this is a background and a powerset, she operates at the baseline human level and her abilities come from gadgets and intense training
- Her codename is Black Widow
- She used to be a villain but isn’t anymore— Natasha is treated as varying degrees of moral, and the redemption angle isn’t always played up, but she’s generally positioned as someone to root for
That might seem like not much but it’s actually remarkably consistent. Compare her with Carol Danvers, who has been through numerous powerset and codename changes in addition to having her emotional memory wiped, or Wanda’s constantly shifting powerset and ethnic background.