So, that random monochrome human in the middle there was CORE!Frisk, who is from an Alternate Timeline of the main universe, in which Sans got tired of Frisk resetting over and over and pushed them into the CORE, similar to what presumably happened to Gaster. Except instead of being shattered across time and space, since they are a human and not a monster, they became omnipresent. They can now manifest themself wherever they want in the form of that human you saw above, and often helps those who watch over the multiverse.