Nicholas Ballard is a chimeric creature born of the experimental interaction between two Devil Fruits of extraordinary power, namely that of the Kumo Kumo no Mi, Model: Arachne and the Shiki Shiki no Mi. Daddy L. Legs used her powers of enchantments in order to transfer various lesser properties from a plethora of devil fruits, torn from their recently slain users, and imbued them within talismans, and then Kosaka Isshin used his powers to weave these fragmented abilities into the soul of a captured pirate, who was then unravelled and put together in the shape of a new individual altogether.
While Nicholai kept his original name, the monstrosity that emerged from this wicked perversion of the laws of nature was someone very different. No longer truly human, but something more.. or less, depending upon ones definition. He was a vile creation of supernatural origins, one who possessed powers resembling a composite of all the different abilities and modifications that had been bestowed upon the original soul. Emerging into this world a grown man, fit with a mature body and a keen mind.
Some might consider Ballard the youngest child of Legs and Isshin, in spite of his highly unusual circumstances. But the bond between Ballard and his parents is more reflective of the relationship between master and servant than of any recognizeable familial connection, at least that is how it seems from Nicholai's perspective.
Eventually Ballard, and others like him created through the same methods were given a name to describe them, namely that of the Nue, monstrous creatures from Wano folklore that possessed the traits of many different animals, much like how Ballard possesses traits from many different devil fruits. Further experiments have been made, but the creatures usually end up possessing only a bestial intelligence and so Nicholai remains a rare success. Even so the Nue are a mighty addition to the Black Widows already powerful military, and Ballard Nicholai aims to see his brethren fulfill their duty to the Spider Empress.
At a glance, Ballard looks the part of a finely dressed gentleman typically seen in a variety of fine-cut suits of various pattern and colors. Taller than most regular humans, Nicholai is nonetheless small in comparison to many pirates, marines and fighters in the New World, including his mistress which completely overshadows him in that regard. Lithely buildt, one can easily make out that his creator didn't intend for him to a frontline combatant, and his overall appearance often leads others to assume that he is a non-combatant member of her crew.
Nicholai's face is considered to be pleasing by many, as it was presumeably shaped to be disarming, given how Nicholai often serves as an envoy sent to collect tribute to his mistress from islands under her control.
However, despite being a nue created to resemble a human, certain physical traits of his are known to stand at odds with that idea. His tail is the first sign that he might not be entirely human, it is a powerful white tail tipped with a stinger that can produce an incredibly powerful poison. His elfine ears are also easily spotted when one gets close to him, and when he opens his mouth to speak, one cannot help but notice a disturbing reality: for his teeth aren't those of a man, they are those of a monster. For they're needle-like and like his tail, are reputed to be venomous. When he closes his mouth these teeth fold back into his gums in order to not wound him accidentally.
Ballard has a theory that the Spider Empress created his body to be both alarming and reassuring at the same time. None of his features are overwhelming on their own, but they make for a constant reminder that he is not a human, nor will he ever be one.