Grimer’s eyes move up to find your pleasure-twisted face. Encourage by your lewd cries and expression, it continues with its plan, an arm of goo squishing its way past your entrance and replacing its tongue deep inside your core. It pushes its way down your full length, then slowly begins to thicken, a mass hardening out from the center of the arm. As it solidifies, the form of the arm becomes more stable, providing you with a pleasantly curved hardness covered with strategically-placed bumps of varying size. The displaced goo has returned to the rest of Grimer, save for a thin film to help keep you well lubricated against the solid form it created.
As this happens inside you, the body of the Grimer has also shifted outside of you. It releases the harness-like grip around your legs and seems to conform to the curve of your cunt - in the back, a smaller prod has formed between your ass cheeks, but this one merely presses against your tight pucker. A look at the Pokémon between your legs confirms what’s happening: the little creature has changed its shape and used Substitute to create the perfect, solid fucktoy, just especially for you.
Grimer smiles your way, excited for what it next. The solid mimic has freed up more of its mass, allowing Grimer to slide up and reposition itself upon your stomach. It flattens out so one arm can reach its substitute, and now another can reach your breasts. Its head appears undecided between the two at first, then ultimately shifts to take one of your tits in its slimy mouth. Your nipple soon becomes subjected to the same abuse your clit once endured, and a moment later the form Grimer created inside you begins to move, its hand moving the thing in and out, in and out. The other hand gives your our breast a tight squeeze, then starts to writhe, massaging more arousal and pleasure out of you with every taunt of the nipple caught in its sludge.
The fucktoy does its job beautifully - it was, after all, adapted perfectly to your core and buried G-spots. Every time Grimer slides the thing in and out of you, its bumps rub against those precious pleasure points, and its tip nudges the opening to your uterus - the whole of your inside is becoming a hot mess of your sex fluid and Grimer’s slime. Behind you, the form’s bump collides against your ass’s pucker in rhythm with the thrusting working your cunt, threatening to invade but never quite making it inside.
The Grimer now has you exactly where it wants you: a tit in its mouth, the other in its hand, and a makeshift dildo fucking you hard upon the alley’s floor. It drives you closer and closer to the edge, showing no signs of slowing down - it has become wholly focused on one thing, and one thing alone. Luckily for you, that thing happens to be delivering you a full-bodied, intense orgasm.