One Koumori's current battle is finished, one way or another, Yukika will take over as my navi.

Name: Yukika.EXE
Gender: Female
Element: Aqua
Subtype: Ground

Appearance: Yukika is about 4'0 in height and she's a very light, skinny net navi, her body is different from any other navi thanks in part to her programing in the fact it's considered made entirely of Ice, (theoretically) allowing her more control over water. She wears the generic body suit that most any navi would, colored a very light blue, as well as wearing a large red wing on her left hand and a wrist band on her right hand, she also has a small pair of insect-like wings in an oval shape that can be seen through, she also wears a visor over her eyes as to hide them from the public. Her body is sleek and well rounded in every aspect with very careful attention to her arms and hands, making her look almost human, abet with her blue crystalline skin and her lack of legs where there is mist on the lower part of her body, giving the appearance that she is actually floating where, in fact, her legs are completely invisible, hidden within the mist. The rest of her body is crystalline, except her stunning, short, blue hair looks as real as most anything found on a human, as if her creators wanted her to have some sort of natural look to her. Finally, her facial details are nearly nonexistent as she has a small crevasse in the middle of her face where her nose is supposed to be and her mouth is mainly just a large chunk of ice missing in a very relative shape of a snowman's mouth, while her eyes are there, they are just completely white and soul-less as if they, along with the rest of her face, were just an after thought in her creation process.

Personality: Her thoughts are generally calm and collected as a combat navi's generally would be, but, due to her data structure and combat processors installed into her programing, as well as a glitch in her core data, she is unable to speak normally and has to "project" her thoughts onto the net or a PET screen via text to communicate to whoever she is talking to. She is capable of compassion, among other feelings, but she generally doesn't have full access to them whenever she wants due to her glitched core data.

Custom Weapon: Her customized weapon can be most anything thanks in part to her body structure that is within reason of course, swords, axes, handguns, etc.

Signature Attack: Iced attack (passive): Yukika imbues an attack of hers with the aqua element once a turn thanks to her unique programing. (Imbue aqua once a turn, 80 points)
(Can I has bump)
Uh... No, you can't. If you're going to be giving Yukika Koumori's upgrades, you're going to have to explain why and how Yukika gets them. I could approve you right now if you wanted to start fresh, but I'm guessing that's not the case.
The why and how Yukika is given Koumori's upgrades is fairly simple.

Koumori's connection to Kedamono is completely severed by a device made by members of Kedamono's family, thus, she and Momo are devoid of help and are easily captured by a large group of navi's under the Kodarashi name. After being captured, Kedamono receives an E-mail with most of Koumori's upgrades, Momo (SP upgrade) is not attached. Kedamono had severed her ties to her family a few years prior to the present, though, her brother, the person who sent the E-mail, managed to sneak his navi into the network to extract the upgrades Koumori had, before being caught trying to save Koumori and having his navi nearly deleted, compressed them, and E-mailed them back to Kedamono, saying that they were even thanks to a favor he owed Kedamono for saving his navi from deletion by their father's navi.

Yukika comes into the picture a little while after this. She manages to crack the security on Kedamono's homepage and, after spying on her for a little while from her on computer, she sends a message to Kedamono, asking to be her navi. Desperate, and unable to make a new one without the technical prowess, and not wishing to go out and buy one of the generic normal navis, she reluctantly accepts Yukika into her PET, installing the upgrades into Yukika and continuing her life as she has, just with a much different navi personality to get used to.
Doesn't really address where Yukika herself came from, but you can cover that on your own time. Good enough, approved, full rebirth get, etc.