Before Papina was born, Lucia-Estrella, and Luan moved with their two year old child, Sanse, to a small community just outside Ebott city. The housing was much more affordable, and the community looked very friendly. In fact, there was a party held to celebrate the newcomers, and everyone pitched in to help the family move in. However, this small village community is not what is seems. The day after, the family was visited by Chara Evans. Chara shut the blinds, unplugs the television, and checked every nook and cranny for any hidden cameras before warning Lucia and Luan of the danger they are in by staying in this community: the Purity of Soul, the executions, the absolute hatred of monsters and monster-humans and those that show support for them, and lastly, the biases they have favoring soul mages so most rules don't apply to them. The family, due to being tight financially, don't have the option to move again; they got lucky with how cheap housing was here, even if they have to shuttle bus into the city. Both parents have to work, both jobs are in the city, and they are both out for most of the day. Chara volunteered to babysit Sanse while they find their footing and rack up enough money to leave; they even said they would help with financial things using their allowance.

When Sanse was 4 and their mother had just given birth to their baby sister, Papina, the Cortez family sat at home as they watched the live broadcast execution of Chara. Chara warned them about hidden cameras that would be watching them to make sure they watched, so the family had no choice. Sanse watched helplessly as their best friend and elder sibling figure disappeared into the depths of Mount Ebott. What Sanse just witness made them hope that they would turn out to be normal when they turn five, that they wouldn't accidentally put a target on their family's backs. Come their fifth birthday, Sanse's worst fear came true. On their doorstep was a plain brown package addressed to Sanse, and inside the package was a pristine white skeleton mask.

Sanse's parents did everything they could to learn about monster-human children, their powers, etc. in secret. They even had tests done at the Ebott City Soul Scientific Research Center to gain a better understanding of Sanse and the powers they posses. Sanse also met up with some of Chara's closest friends from the city, and they helped Sanse practice their powers in secret. All the while, the family kept Sanse's 'gift' a secret, even from Papina, as the young child proved from a very early age to be too curious and too talkative for their own good. For Sanse, that would be a bit tricky, as after school, they would have to pick Papina up from daycare and go straight home, only to then try sneaking off to train or to then be picked up by their mother and taken for testing.

They kept this up for four years, until Papina's fifth birthday. Another package, another skeleton mask. Papina always wanted to know what her family was hiding, but in that moment, with all the answers being flooded into her brain, she regretted ever wanting to know. The next few day, Papina was also taken to the same research center for testing. Sanse also gave Papina the rundown on the routine: while at the village and anywhere the PoS could be, they are to hide their masks. But they also can't leave their mask at home, because it will appear by their side when they leave it a certain distance away from themselves. So they are to use backpacks or tote bags wherever they go so they can hide their masks. When Papina told Sanse that they were scared and that they wish that they were brave like them, Sanse taught Papina that bravery is being able to acknowledge that you're scared, but still face what scares you so.

One day, during school, Papina witnessed a person being bullied by two older kids. Paps had also been bullied by these same people before, but they mysteriously left her alone one day (thanks to Sanse's secret private 'talk' with them). She didn't want to face their wrath, but she also couldn't stand by as someone else faces it instead. Pap's recalled what Sanse told her, and steeled herself. She took out her mask and put it on, and, using some abilities she learned from the research center, she stopped the bullies. However, the child she saved was a child from the village.

The Cortez family tried to run. But they were soon caught and taken to the top of Mount Ebott. For knowingly hiding the truth twice, and both children having full knowledge and hiding it as well, they were all publicly sentenced to death. They forced the children to wear their masks and tried to feed them the poison broth Chara warned the parents about all those years ago, but Sanse got on the defensive. And so did their mother.

Lucia being a patience soul mage froze almost all the PoS in place, except the Integrity soul mage council elder's soul in the crowd. They went to attack Sanse and Papina, but Luan took the damage. He fought with the elder while Lucia took Sanse and Papina in her embrace, told them that she and Luan loved them, and then pushed them into the hole. Sanse, held Paps close to their chest as they fell so only Sanse Saw their mother being stabbed in the back. Only Sanse saw their mother's beheading. And when they fell to the bottom, only Sanse saw their lifeless bodies drop down with them.

Torel found them as he heard Sanse's screams for help. He quickly led the children away and cremated the bodies, giving the urns and Lucia's soul to them. The two children would spend a year in Torel's orphanage before being reluctantly adopted by W.D. Gaster. Papina was happy to have a parent again and was eager to get to know their new mother. Sanse, however, could tell that Gaster had ulterior motives, and made sure to make her life hell. After some encouragement from Papina, and Sanse finally getting their answer (which yes, Gaster had ulterior motives to study Sanse and Papina, as it should be impossible for Soul Mages to give birth; Soul Mages are commonly sterile), and some bonding time, Sanse warms up to Gaster. Gaster teaches Sanse and Papina, and even takes Sanse to her work. They also occasionally are babysat by either Torel or Asgore.

One day, during one of Sanse's more private studies, Sanse proposes an experiment to not only give them a black soul, but also fuse both the human and monster souls together. Gaster initially refused vehemently, but Sanse blackmails her with telling Asgore and Torel about her black soul and all the lies she has told them and noted in her personal journal. Gaster gave in and experimented with Sanse. There were several hiccups, and several resets, and Sanse nearly dying several times over due to unknown DT overdosing, but Gaster used parts of her own soul to equalize the attributes.

With Sanse's black soul, they gained little extra abilities from each attribute: Faster attacks and consistent damage per second with Bravery, a skull familiar that holds their blast bullets and acts as a shield from time to time with Perserverence and Kindness, also general telekinesis with Perserverence, and gravity manipulation on the soul with Integrety. And because of Sanse's fused soul, their human body and skeleton body find a happy medium between the two. Sanse no longer needs to use their mask to transform between the two forms. However, the drastic changes happening to Sanse's body got Papina concerned. She did her own investigating. The thing that concerned her the most is that Sanse can fully remember resets, and so Paps goes to Gaster with the demand to giver her the determination trait so that she may also remember resets so Sanse wouldn't be alone. Gaster again tried to refuse, and she believed she would succeed this time, but Papina just sang at the top of her lungs, trapped Gaster's room, and made it where Gaster could barely sleep or go about most of their day without hearing the obnoxious singing. Gaster soon agreed out of desperation. Gaster did take this chance to see what would happen if a piece of her soul was added to another without all the other attributes also being used. So Gaster injected a DT mixed with a fragment of her soul.

When Sanse was 13 and Papina was 9, Gaster fell into the core and was erased from everyone's memories, except theirs. They didn't know this at first, so it comes as a surprise when no one they talk to remember the late scientist. Gaster was well known, so it would be impossible for people to not know her. Not knowing what else to do, they turn to Torel, who, like everyone else, doesn't remember Gaster. He believes that he and Asgore had adopted them. Sanse and Papina don't reject any of this and just try to move on. They spend several years trying to move on from Gaster's "death". Sanse takes to learning all that they can about what happened to their adoptive mother, and Papina opts to being even more of a beacon of positivity, especially for Sanse. They each try to keep something of Gaster's as they move in with Asgore. Gaster didn't have very many things of hers except books. Papina goes out to the Echo pumpkins and picks out Gaster's pumpkin; it lead her to a secret compartment book where she puts a small picture frame of Gaster, her, and Sanse at the pumpkin patch in it. Sanse takes Gaster's Journal of Scientific Inquiries.

One day, Sanse tries what they had set out to do by turning their soul the way it is: Break the Barrier. Sanse tried and tried, but the power needed was much more than what their soul could take. Asgore had to drag Sanse away before they could do permanent damage to their soul.

A year later, the River Person summons the two. R.P. had created a new soul mage child, named Patricia, using their mother's soul. River Person told the two that the very nature of this child shouldn't be told freely. Sanse and Papina both decided to keep this a secret. On the subject of keeping things a secret, Asgore had them keep Patricia's Soul Mage status a secret from everyone. She's afraid of another PoS member taking away another child's life just to keep people underground. They too promised. For Sanse, they would carry these promises and secrets to the grave, the only one that would hear about it would be Chara. For Papy, she's not so good with secrets; that's why her best friend Flowey can hold them for her!

Over the next 7 years, Sanse takes odd jobs here and there to accrue enough money to pay for Gaster's old house in Halloween towne. Sanse also took up Gaster's position on overseeing all the different scientific departments. On the tale end of the last year before moving out, Asgore offers Sanse a more permanent job. She offers Sanse a job as an undercover sentry, and secret judge. She knows Sanse's soul works different than most, though she doesn't know how. She knows Sanse can see people true essence, and no secret can be kept from them. And with Asgore growing more paranoid by the day about secret PoS followers being underground, having Sanse as the judge would give her piece of mind for everyone's safety. Sanse took the job without hesitation.

With a more permanent and stable form of income, Sanse sticks with sentry duty, scientist, and occasional judge if anyone wants to see Asgore. And while Sanse thinks Papina doesn't know much about it, Flowey makes sure to keep Papy updated. Papyrus in return keeps everything tidy, takes care of Patricia, learns to cook, and in general tries to keep a positive atmosphere for her family.

When Alphys takes over as the new head scientist, Sanse takes up selling candy apples as a cover up for their three sentry stations.

With the rise of Mettaton, Papina became really into cosplay. Mettaton became her idol, and perhaps Papy formed the tiniest crush on the star.

One day, there was a massive raid from the PoS members. Several empty souled humans purposely fell down into the Underground with the purpose of eradicating everyone. They were stopped quickly though before they had a chance to properly storm the Fallin paths into Halloween towne, thanks to the royal guard and secretly Sanse. However, many were severely wounded in the process, and several were 'Falling Down'. It would be only a matter of time before Sanse finds out about Alphys' failed experiment that caused the Amalgamations.

On Papina's end, she saw how brave an selfless the guards acted. No civilians were hurt, only the guards. They were so cool! Papy made herself a cosplay of her own guard outfit. And after a party where someone told Paps that she should try out for it, she took that idea and ran with it! She took the potential to become a guardsman very seriously. She used actual armor for her outfit, while still keeping it fashionable. Papina goes to Undyne's house to volunteer to join the Underground guard. Undyne turns her down. He knows who Papina is. He knows how strong she is from watching her in Waterfall during his patrols. However, he also has close ties with Sanse through their respective jobs, and Sanse would end him if he let Papina join. However, after a full night of Papina guarding Undyne's house and still having the energy to excitedly greet him as if she too had a full night's rest, Undyne decided to "train" Papy.

For the most part, they attained normalcy. Hopefully, everything will remain peaceful. But that simply isn't the case, not when Frisk enters the Underground.