Well what if later on in the RP when Eve gains access to the Code: Exotic she also gains access to a series of other codes (that won't be changing her form or such, but could be used as programs or such to do different things.)
One of those could be something that Eve and Cortana could accidentally access during a dramatic moment (like a major fight) where someone the code allows Eve's and Cortana's bodies to merge (or a digital copy of Cortana) to merge with Eve.
As soon as such an event happens Eve's body immediately transforms into something incredibly powerful.
I even thought of a decent description of sorts for it in this RP.
It was clear as day when it happened... Eve and Cortana were busy trying to infiltrate an enemy flagship. However they were caught... surrounded by a seemingly never ending swarm of enemies Eve and Cortana tried to fight them off, yet where being overwhelmed much like they were when Eve accessed her Exotic Code.
That's when it happened. Cortana's digital self had accessed Eve's systems in hopes of finding a more powerful code. That's when she managed to tap into an ancient code. An extremely powerful, but locked code . However it could not be accessed by even Cortana due to the codes raw power. Eve needed help for this Code to activate. As such Cortana's digital self merges with Eve's systems. Causing an temporary evolution in Eve's abilities. Eve grew far more powerful, far more intelligent. She had Cortana's knowledge, and power along with Eve's very own power. They had become... the Battle Seraph!
Basically this would be a temporary form. Basically Eve would gain an increase in her own abilities. Able to perform a wider variety of attacks with photons and such, as well as having Cortana's hacking abilities and such while in this mode. It's only temporary due to it's power, and has no real downsides except that it's temporary, and that Cortana's digital self must literally temporarily merge with Eve (it would be a forced kind of temporary. After some time they would "defuse" from each other, and go back to normal.) Cortana's physical self would also gain the transformation during this temporary amount of time. Allowing her to fight along side Eve while in this fused state. However once the fusion wears off Eve and Cortana's digital self would separate, and the transformation on Cortana's physical self would also wear off.
It would definitely be something that would be introduced late into the RP, way after Eve access's Code: Exotic, and the advancement Code: Nemesis. Battle Seraph mode would basically be stronger and faster than Code: Nemesis.