In DND mythology their is the concept of the Illithid adversary.
When an Illithid undergoes ceremorphosis, it can occasionally take on some elements of the absorbed host creature's former mind, such as mannerisms. This typically manifests as a minor personality feature, such as a nervous habit or reaction (e.g., nail-biting or tapping one's foot), although the process that determines the type and number of traits so inherited appears to be stochastic. Some adult Illithids have even been known to hum a tune that its host knew in life. Usually, when a mind flayer inherits a trait like this, it keeps it a closely guarded secret, because, were its peers to learn of it, the Illithid in question would most likely be killed. This is due to an Illithid legend of a being called the "Adversary". The legend holds that, eventually, an Illithid larva that undergoes ceremorphosis will take on the host's personality and memory in its entirety. This Adversary would, mind and soul, still be the host, but with all the inherent abilities of an Illithid.
