Codebreaker!Frisk

Codebreaker is the sole survivor of their universe's collapse, now  existing as an outcode determined to save others from a similar fate.  They're very quiet, typically signing or writing instead of speaking  unless with friends. The loss of their entire world weighs heavy on  them, but they do their best not to let it drag them down.

Following your typical pacifist Undertale timeline, Frisk backtracked  through the underground, mainly ensuring everyone knew the barrier had  been broken. On their way through Waterfall, however, something changed.  A door they hadn't seen before, sticking out like a sore thumb.  Curious, they of course entered, and were greeted by a monster they'd  never seen before.

They got straight to the point - "Hi, I'm Frisk, yes I'm human don't  worry about it, the barrier is broken you're free isn't that cool" - but  it didn't matter. The vaguely-skeletal monster didn't care about  freedom in that sense. The only freedom he needed was from his own fate.  The only thing he needed was a soul. And here, before him, stood a soul  ripe for the taking.

Gaster lunged for Frisk, who frantically tried re-explaining that a  fight was actually not necessary anymore. Seeing as talking wasn't  working, Frisk ran, though it was already too late. They didn't know it  yet, but this battle had left its mark on their soul, and by extension,  the timeline.

Gaster's attacks left a dark splotch on Frisk's soul, corrupting it.  While Frisk themself was largely unaffected by this at first, the  timeline was not so lucky. Being directly tied to Frisk's determination,  and thus their soul, this corruption spread to the very fabric of their  universe, turning it into a timebomb.

It wasn't all bad, though. About a week after the incident, they  accidentally discovered a new ability to teleport. They brought that up  to Sans, who helped them hone the skill a bit more over the following  few months. This did require them to confide in him about their  encounter with Gaster, which was not a pretty discussion. This  conversation ends with the revelation that their universe was doomed,  and there was nothing they could do to save it. Frisk's soul was too  corrupted to risk a reset, and the timeline was too impacted to assume  killing them would fix it, as Flowey suggested when he found out.

Completely ignoring their impending doom, Frisk took their newfound  capability for magic and asked Toriel to teach them magic like hers.  They did have to bullshit an excuse as to how they could use magic for  that, but it was definitely worth it. While they never truly mastered  fire magic, they're decently skilled at healing.

About a month before their universe ended, Sans gave them one of his  spare jackets. It wasn't meant to be a big deal, but seeing as he has  since ceased to exist, it became one posthumously. They basically always  had it on them before losing their universe, and especially  now, they never take it off.

The death of a universe can be many things. It can be dramatic, with  flashy explosions or apocalyptic catastrophes. It can be slow, fading  into darkness, things just ceasing to exist over time until nothing is  left. Codebreaker's universe ending was none of these things. It was  simply there, and then it wasn't. And yet, for some reason, Codebreaker  remained.

Floating alone in a timeless dark void, they eventually found a glimmer  of light, so small it would never be noticeable if anything else  existed. Reaching out, they felt the now familiar sensation of  teleportation, finding themself in a new space beyond their universe.  They didn't entirely know how they knew that, but they did. This  was also when their scleras went dark, and their glitches appeared. It  would be a while before they found that out for themself though.

Looking around this new multiversal space, they could see hundreds of  universes surrounding them, each its own isolated existence. The sight  filled them with a burning feeling they were ever so familiar with;  determination.

This determination to protect is what keeps them going, and they do  everything in their power to protect other universes from being  corrupted as theirs was. On the occasion a universe they come across is  too far gone, they evacuate those who can be saved to the void where  their universe once was, the space slowly filling in with bits and  pieces of the universes they've brought refugees from.

They are a bit more awkward than other Frisks may be, and struggle with a  great deal of anxiety and survivor's guilt. They know how detrimental  even the smallest mistake can be, and when dealing with data corruption,  it can be incredibly difficult to figure out what is  salvageable. Paired with the cases in which they have to leave people  behind, their role can be incredibly taxing, but they hold tightly onto  their determination to save what they can.