This monster does not have a name. It hides inside the waterfall where you obtain the old tutu. The Echo monster mirrors the person's mental state that echo's into the Echo monster's form. This happens in an AU where Flowey accompanies Frisk, Frisk tries to do a pacifist route but Flowey keeps gaslighting Frisk, he keeps saying that this world is kill or be killed, so Frisk breaks under the pressure and does a geno route hence why there are vines that are breaking Echo Frisk.
(If you're asking, yeah this guy's inspired by the Mirror monster from BnP)
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