Isekai where instead of reincarnating as a human in a fantasy world, a neet with a trap fetish awakens as a familiar in a digimon/SMT game world where his master is a deceptively cute boy. All too familiar with what awaits him, he prepares himself for usual monster raising mechanics which seem like they'll end up altering his very being to its core but to his surprise he's been granted with the ability to choose to what degree fusions and evolutions will alter him letting him avoid having his ego collapse into that of a mindless slave.
Tragedy strikes however as a mysterious group of sentient humanoid monsters destroys their village leaving only his master as its sole survivor. He manages to fight one off the female monsters off but is left badly weakened and in a lethal daze. Afraid to lose his last living acquaintance, the boy forces a fusion between his familiar and their would be killer.
It works but the MC now has a female humanoid body with aspects and memories of his other half bleeding over into his self. The influence almost overcomes him and nearly pushes him to kill his master but his love of cute little girls(boys) wins out allowing himself to stay control. This new form allows him to pass himself off easily as a young woman with the right clothes to hide the inhuman qualities and the fusion weakened the connection enough for him to disobey his master unpunished but instead of taking the chance live freely, he makes a bargain via a slave contract with his master to get revenge on the monsters that destroyed their village in exchange for the right to groom the boy into his ideal trap.
With a mysterious group and vengeance to chase after, the two of them will travel the lands and raise each other up to be what they want the other to demands of them as they begin their descent down into a spiral of compromises that will leave their beginnings and endings worlds apart.