Is the small maid in the color page.
She's part of the prologue story, a human working as servant among demons trying to find a chance to avenge her dead family that die when demons attacked her village.
In fact in the game she's the one who finish him off after he run away from the hero party.
After which she'll go jump off a cliff and kill herself.
MC bring back the elf girl to ask her about this kinship process.
Which she explains it only work on demons, more specifically on those with demonic elements.
So MC hatch the plan, using a 'monster soul' that's just a stat booster item in-game and part of the betrayal glitch condition.
He hypothesize that this soul once consumed will impart demonic elements to the user which allow for turning them to demon and then turning them to his kin.