Jack-O' Valentine

Jack-O' Valentine or simply  Jack-O'  is a character from Guilty Gear series, she makes her debut on Guilty Gear X Plus appearing on Sol's Ending and her playable debut was in Guilty Gear XRD -REVELATOR-.

Profile
She one of the three Servants of That Man alongside Raven and I-No.

Because her awakening remains incomplete, she can't function without her mask and toffee. Her past is shrouded in mystery, and her own memories are vague. Because she's in an unstable state, the tone of her voice changes rapidly.

Background
In 2074, Justice's body and consciousness were compromised by the Universal Will during her activation test when she was linked to the Backyard. After replicating Justice's DNA and using it to create the Variable X Gears to assimilate the Earth into the Backyard, Asuka R. Kreutz ordered a Manual Override that transferred command of Justice's control systems over to himself and had Justice unwillingly fire her Gamma Ray at Japan, the source of the Variable X phenomena. Because her mind had been warped by the Universal Will's influence, her memories and soul became altered. Asuka extracted a partial replica of Justice's memory core and used it as a base to synthesize Jack-O' Valentine. She is "Half-Aria" as she refers to herself, due to being born from an incomplete copy of Justice's memories and genetic material. She is the ultimate Valentine model created to fuse with and complete Justice.

Guilty Gear X
She was briefly shown as a hooded shadow in one of Sol's ending alongside with Raven, I-No and Asuka, but then she was put to sleep until the time for Justice's revival.

Guilty Gear XRD -REVELATOR-
Jack-O' is awoke prematurely by I-No to start with the plan of resurrect Aria, but for being asleep to long she escaped to the outside world to have some fun. While she was outside Jack-O' fought against a lot of characters, those were Bedman (who injured her legs), Axl, I-No, Ramlethal, Sin and Sol who ended up defeating Jack-O' and demanded her to show him her face. When Jack-O' revealed her face, Sol is shocked by the incredible resemblance with Aria and then Jack-O' proceeds to explain who she is. By the time she finished her explanation Jack-O' disappears alongside Raven, awaiting for Asuka to come back to continue with their plan.

In the meantime Jack-O' and Raven were watching Sol and talking with each other until Asuka came back and proceed with the plan. He and Jack-O' were teleported to Illyria Castle to explain their plan and reveal the truth about Sol, Justice, the Universal Will and the Seeds. After all was explained Jack-O' went with Sol, Sin, Ky and Johnny to merge with Justice while That Man prepare the Saint Oratorio (a type of unlimited Magic energy) to fuse Jack-O' with Justice.

Once they reached Justice and Ariels' location, Jack-O' extracted Elphelt from Justice and prepared herself for the Saint Oratorio, while Sol, Ky and Sin fought against Ariels. After an intense battle and the intervention of Raven and Ramlethal, Jack-O' receives the Saint Oratorio, which also severely weakens Ariels, who was in the path of the beam. However, the energy of the Saint Oratorio wasn't enough to fuse her with Justice because of the trauma that she was still suffering, and the only way of merge with her is to get hit by a greater amount of energy than the Saint Oratorio. In that moment, Third King Daryl appears with a fleet of airships equipped with Collider Cannons with the intent of eradicating Justice while she was still incomplete. After teleporting everyone but Sol to the ship, Daryl fires the cannons to destroy both him and Justice. But in that moment, Axl compressed time in order to give Sol a chance to receive the Saint Oratorio with his Junkyard Dog MK III and maximize the energy with his Dragon Install, hitting Jack-O' with it and forcing the merge between her and Justice, which creates a new version of Aria in the end. She was last seen being carried by Sol as the new Aria.

Guilty Gear Strive
Though Aria fully regains her soul, the old Aria's consciousness has yet to return from behind Jack-O's. Three weeks later, Jack-O'—wearing her halo although she doesn't need to—is still waiting, but starts working with Sol as a bounty hunter. After a successful bounty run, Sol is recalled by Ky to the castle, and learns that I-No took something from Ariels, but Sol takes Jack-O' on a vacation to Iseo instead. Their journey is cut short by Axl's warning, I-No letting herself be capture, and a highway encounter with I-No's ally Happy Chaos that involves Nagoriyuki, Chipp Zanuff and Anji Mito. Back at the castle, Sol and Ky interrogate I-No, but she reveals nothing.

Instead, Jack-O' tells the duo of I-No's origins as a Magical Foci, and that she is incomplete. Thinking that I-No may be after Asuka's Tome of Origin, Sol decides to take up President Vernon's offer to be a knight at G4. Jack-O' remains behind to stop I-No if needed; in truth, she is projecting herself onto I-No, wanting to learn about her. "Aria" tells Jack-O' to understand the value of her own world, otherwise Aria will disappear. As the summit starts, Jack-O' asks I-No what is her wish, and I-No admits to wanting to get out of her "empty cage", not without making Jack-O' question if there is a place in the world for her. Ky, hearing their conversation, has Jack-O' accompany him to G4 aboard his airship. On the way there, Ky tells Jack-O' about humans' "necessary frivolities".

They arrive after Sol, Vernon, Asuka and their allied forces regain control of the airborne White House, after ejecting Chaos on a escape pod. There, Asuka informs them that he will use the aircraft to go to the moon, and proceeds to remove from Sol his divine seed, restoring his humanity in the process. To their surprise, Chaos is still onboard, and gets a hold of Asuka—who is one with the Tome—; Chaos thus merges with I-No, ascending her to godhood. After she knocks Sol unconscious and proceeds to grant "freedom" to humanity, Ky asks her to stand back, but Jack-O' takes one last look at Sol's face and then forcibly enters I-No, to turn her human at the cost of her own life and fulfill her role of protecting the world, in spite of Ky, Axl and Asuka's pleas.

Before Jack-O' can complete the process, Sol asks her not to do it, even if that may save the world, but she tearfully admits—thinking it is only Aria whom he wants a future with—that Aria isn't coming back. Sol says it doesn't matter, but that he won't let Jack-O' go no matter what. Deciding that she wants to be by his side, Jack-O' then comes out. With Jack-O' safe, Sol then has Ky distract I-No while he aims with the Outrage. Thanks to Nagoriyuki, he hits I-No at her weak point, killing her and saving the world. In the aftermath, Jack-O' decides to abandon her halo and live together with Sol—now simply Frederick—at Iseo.

Guilty Gear Dust Strikers 2
Jack-O' appears as a playable character.

GearBlaze: Birth of the Calamity Carnival
Jack-O' is one of the characters from Guilty Gear side.

Project X Zone 3: Sacred Destinies
Jack-O' appears as a Boss Unit.

Her story in the game is an alternate re-telling of her escape to the outside world and her encounter with Sol, Sin, Ramlethal, Raven and I-No.

She first appears in Chapter 5: Playerdimension Neptunia where in middle of her travel to have some fun, she encounters Reiji's group in The World (.Hack//Series), engaging in a battle that ends with her defeat and retreat.

She eventually faces Sol Badguy in the Fujisakura Altar (Super Robot Wars OG: Endless Frontier), where Sol and Ragna demands her to show her face, with her refusing to do so during her next appearances in the game.

Finally, after being defeated in Chapter 36: This is NOT my Final Fantasy!, Jack-O' promises Sol to reveal her face if he apprehend and fights her in their world once he is done with Ouma, finally returning to her world and leaving their promise at hands of Sol.

Guilty Gear vs BlazBlue: Multiversal Cross Assault
Jack-O' appears as a DLC character.

Trivia

 * Like the original Valentine, Her blood is Type O. Curiously, younger generation Valentines such as Elphelt and Ram cannot be analyzed. This could be because they are technically less human and improved Valentines.