Bowser Memory Neuron
From the Super Mario Wiki
Bowser Memory Neuron is a character featured in Mario & Luigi: Bowser's Inside Story who shares resemblance to Napses. When Mario, Luigi and Starlow come to the Memory Banks to restore Bowser's memory so he can get the second Star Cure, the neuron immediately assumes they're viruses and transforms into Bowser Memory ML to finish the bros. After the bros. beat them the cell takes on a new form called Bowser Memory P which is the memory form of Peach and the neuron finds the safe combination. After the bros fix the combo, Bowser enters it and gets the Star Cure only to later be betrayed by his minions and locked in the safe. It can be seen any time later in the game in the Memory Banks, but cannot be interacted with whatsoever.
In the Nintendo 3DS remake, Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey, it is not seen after revisiting the room where it was fought.