Debuting in the lesser known spin-off Guilty Gear Isuka, A.B.A is a homunculus from the mansion-laboratory of Frasco, located deep in the Northern mountains. The manor was owned by the alchemist and researcher Paracelsus, who spent his time experimenting with methods to create artificial life. A.B.A was the result of these experiments, leading to Paracelsus being taken away by the military shortly after her creation but before she first awoke.

This left A.B.A in isolation, trapped in the abandoned mansion for the first ten years of her life. She eventually managed to escape Frasco, only to be faced with the reality that she had no clue how to exist in the outside world. She found a fascination in collecting keys as a method of escape from her sadness.

During her 10 years in isolation, A.B.A stumbled across an ancient war relic. She fell in love with it immediately due to it being shaped like a key and declared it her partner; her husband.
The relic known as Flament Nagel was once a beast roaming the wasteland during the Crusades. In this form, he met the vampire Slayer and they exchanged blows in a devastating battle. Slayer found out Flament's body was merely a vessel, destroying it and leaving only the key-axe behind. The flask alchemist Paracelsus found this war relic on his travels, taking it back to Frasco to research it. Eventually, he would be taken away after A.B.A's creation.

A.B.A became infatuated with Flament, renaming him "Paracelsus" after her creator. Her lifelong goal from this point on would become to find Paracelsus an artificial body like her own.
Paracelsus does not reciprocate A.B.A's feelings and has no interest in being given a body of flesh; however he does worry about her and enjoys fighting alongside her in battle.

Paracelsus has two states, his standard form and his fighting spirit form. In the context of Guilty Gear Accent Core +R, these are referred to as "Normal Mode" and "Moroha Mode" (諸刃 lit. "double-edged") respectively.

A moment in A.B.A's Story Mode where she meets Slayer, with Paracelsus calling him a "friend" calls back to Paracelsus' fight with the vampire. This marks the third time Paracelsus and Slayer have met. Their first encounter was their battle in the wasteland, but their second encounter was when Slayer was tasked with checking out the abandoned Frasco mansion years prior.
This is when Slayer first met A.B.A, challenging her and Paracelsus to a duel; if they put up a good fight, they may go free. This fight resulted in A.B.A losing an arm, but her nature as a homunculus allowed her to reattach it easily; Slayer also commented that Paracelsus' teamwork with A.B.A was more effective in battle than his original vesseled beast form.

In the first of her two non-canon Accent Core +R endings, A.B.A manages to get Paracelsus a body by taking him to the Post-War Administration Bureau (P.W.A.B).
After finally achieving her goal, she chains Paracelsus up to the wall of a meat locker to keep him from running away.

In her second ending, Paracelsus' consciousness is transferred into a body of a Robo-Ky model that ends up decapitated, along with the device used to transfer his consciousness being broken.