Core Frisk is basically a super hero with a sad backstory. Their most defining feature is their
monochromatic color scheme. This is because there is a condition called "CORE Bleach" where you go too close into The CORE. Their second most defining feature is their pitch black "eyes" (note that I quoted eyes). This is because Core Frisk doesn't have eyes. Oh yeah, and it can also read minds and decipher the coding of a Memoryhead.
It all started as a normal-looking Frisk that has been through a civil war, and Frisk is in danger of death. They want to find a safe place until they accidentally fell into the Underground. They then got a happy ending, then reseted to get another happy ending, then rested again to get yet another happy ending. Sans got mad because he hates timelines jumping back and forth, and then there is the ultimate fate. Frisk has been dunked into the CORE and then become this.
As of now, Core Frisk lived an interesting life, as they created something called the Omega Timeline, the timeline for the sole survivors of basically anything from Genocide to having their whole universe being destroyed with them. Right now, Core Frisk and the rest of the YTP characters are helping to bring their world into the Omega Timeline, so that they will live on and on. Besides that, Core Frisk has a BIG fear of guns and stuff sounding like shooting, as this bring back to their flashback about the civil war they're in.
There is a suspicion about if it's dead or supernatural. Some say dead because falling into the CORE usually results in death, but some say supernatural as they manage to survive the CORE. No matter what, they are the hero of the YTP universe, preventing it from being destroyed by building that world with the rest of the YTP characters.