Non-binary. Xe/xir prounouns. (Male pronouns OK.)
Biologically, one can say that Viper appears female, but only through a few hints. Viper has a short stature, a small body, long hair, and appears on several occasions, most notably so in adult form during the Arcobaleno battles (manga), to have full lips, a feminine trait.
However, the very nature of the clothes that Viper wears makes it difficult to attach a gender to the person. Viper wears a long, rather-flowy cloak, and a hood that conceals the majority of the face, including eyes, from public view. This choice of attire is done primarily to retain an air of mystery, something that Viper, as an illusionist, relishes. Viper likes xir secrecy, as it proves to be advantageous on countless occasions, especially when working in the mafia, whether it is with the Vongola or Variaâthough Viper has, in the past, taken up other jobs as well.
Anyone who sees Viper in public is forced to make xir own assumption about Viperâs gender, due to the obscurity of biological features that do not obviously adhere to traditional gender roles. Viper does this deliberately and likes the confusion.
Viper never enjoyed being a girl. Growing up, Viper never felt as though xir brain worked like a girlâs, even when xe ignored the stereotypical girly-girl roles; xe just never viewed things the same way. Xe never liked the things most girls seemed to satisfy themselves with, and getting teased for it eventually caused xir to grow uncomfortable with xir gender. During xir teenage years, Viper began to introduce xirself as a boy. It helped, at the time, to identify as a boy instead of a girl, and it was easy to act like one and make himself look like one; additionally, xir ability to manipulate xir appearance with illusions always helped.
Being expected to be extremely masculine, however, also caused problems. It didnât suit xir as well when xe got older. Xe decided that neither male nor female social roles suited xir. Being agender appealed to xir a lot. By xir twenties, Viper was qualified to be a master of illusions, so it was easy for xir to brush gender off as something irrelevant.Â
Viper is an illusionist, endowed with the ability do become whatever xe wanted. That was nice. That way, it didnât matter what gender xe was. The thought satisfied xir; xe was comfortable with xirself.Â
Gender neutral pronouns like xe/xir suited xir wonderfuly, but after calling xirself male for a number of years, those didnât particularly bother xir either. Xe didnât particularly feel drawn towards male pronouns, but xe isnât the type to care enough to correct very many people.
In the end, Viper was simply Viper.