Eunice

Eunice, aka The Unitrix, is a female "Human" from The Transmogrification of Eunice. The Unitrix is a prototype Omnitrix which can absorb alien DNA in the device separately one for each species intended to store the DNA in separate containers. Ben had a big crush on her.

Backstory
She is the Unitrix created by Azmuth, which is a prototype of the Omnitrix. Sunder was hired by Azmuth to recapture her. During the scuffle with Sunder in a muddy area, Azmuth arrived and cleared things up. He stated that Eunice was actually a Unitrix, a prototype of the Omnitrix when Azmuth wanted to preserve alien DNA. Following a brief argument, Ben allowed Azmuth under a compromise to take Eunice with him as one of his assitants as Azmuth stated he can use a hand on taking care of Primus.

Azmuth states that when Gwen touched the pod containing the Unitrix, her human DNA was absorbed, and then randomized (which explains why she has blonde hair) creating Eunice's humanoid body.

Appearance
Eunice's mobile form is a beautiful teenage human girl. She has shoulder-length blond hair with a single bang, and bright green eyes, like Ben and Gwen. When they first found her, she was completely naked, then she wears only Ben's bright green long-sleeved jacket. Afterwards she dons a pale pink tank top, white shorts, and brown flip-flops, which she borrowed from Gwen. In her technology form she is the Unitrix, a core similar to the Omnitrix and Ultimatrix.

Powers and Abilities
As a prototype Omnitrix, the Unitrix (and therefore Eunice) is capable of storing and recalling DNA of different species. Unlike the later versions like the Omnitrix and Ultimatrix, she is not connected to the Codon Stream nor Primus, limiting it to a single DNA sample. Eunice was also shown to be able to absorb DNA and powers of several animals (a rabbit, a bear, and a snake) and then to manifest traits of these animals (greater jumping ability, increased strength, and abnormal flexibility respectively) without changing form. The powers and abilities she absorbed from aliens and animals only last for about ten minutes. There is also a green rectangle on the back of her neck, and when Sunder touched it, it turned her back to the Unitrix.

In addition, local wildlife seemed to be attracted to her to the point where normally shy animals would gather around her, and render more dangerous animals (like a bear or snake) docile towards her and others (this capability was likely included by Azmuth as a way for the Unitrix to easily obtain DNA samples from them likely to protect her and the item from danger since its only temporary and the Unitrix can only contain one DNA sample).

Trivia

 * An interesting fact is that Eunice is almost nothing like Gwen, she has blond hair (opposed to Gwen's orange/red hair), and she looks more similar to Jennifer Nocturne than Gwen (despite them being completely different people) this is because her DNA is randomized. and she never displayed any Anodite powers (because there's no DNA for Anodites).
 * Unlike the Omnitrix and Ultimatrix, she is not connected to Primus nor the Codon Stream, as the Unitrix stores DNA samples that are taken by the device.
 * As her mobile form is made of human DNA, it would technically make her as Azmuth's first human assistant, a possible replacement after Azmuth's former assistant Albedo went rogue and possibly for his need of new assistants.
 * Eunice is one of the few characters to be shown basically naked, similar to Kevin each time he reverts from mutant form to normal. Although in Eunice's case, she was actually fully naked whereas Kevin always had something on his bottom half.
 * Eunice bears a resemblance to Jennifer Nocturne, though being younger and more beautiful, with bright green eyes and not blue.
 * There is a possibility that because of the intent of the Unitrix more than one device of the same kind could possibly exist and their could be others in existence maybe other prototype models as well.
 * As the human DNA being sampled and stored she has in a way technically fullfilled Azmuth's original function for the Unitrix. So you can say Eunice is now a preserver for the human race. Despite the device being an prototype model of the Omnitrix even though the Codon Stream already has Human DNA being preserved in it.
 * It was confirmed by Dwayne McDuffie that whatever she absorbs, she only has the powers of it for 10 or so minutes, much like the Omnitrix and Ultimatrix.
 * Dwayne said that we will see why Azmuth said that Eunice aka the Unitrix was so dangerous.
 * Eunice's mark on the back of her neck resembles the brand of a Pulse l'cie from Final Fantasy XIII.
 * The reason the Unitrix landed on Earth is because it was seeking a Omnitrix core like itself and the Ultimatrix technically has a Omnitrix core.
 * Despite the Unitrix having been created on Galvan Prime, Eunice as a person was born and created on earth.
 * Eunice is technically related to Ben and Gwen since she has Gwen's DNA. This was said by Azmuth explaining that the Unitrix sampled then randomized Gwen's DNA into her own unique human DNA and created Eunice.
 * Since Anodites don't have DNA, Gwen's DNA is fully human, meaning Eunice does not have Gwen's Anodite powers and her body doesn't contain anodite life energy.
 * Eunice at the beginning, her origin was kind of similar to Supergirl's arrival on Earth; she appeared in a space pod, she was fully naked, and she didn't know what was happening.
 * the diffence between Supergirl and Eunice is that Eunice wasn't worried, and Supergirl was, Eunice isn't as strong as steel and supergirl was.
 * Her blond hair could be the fact that Gwen's hair traits are Heterozygous Orange (Oo) leaving blond hair as the recessive gene.
 * She is first shown in The Transmogrification of Eunice.
 * It is possible if Ben or Kevin had touched the space pod Eunice would have been male.