Morrie

'Morrie' looks like an innocent rag-doll, but technically it only exists inside the mind of David Haller.  He had created the doll as a means of giving control of his body to only one personality at a time.  His mind had become flooded with hundreds of different personalities and possession of the doll would mean that only one personality could be in control of his body at a time. 
 
David entrusted possession of the doll to Marci Sabol, an innocent little girl whom one of his personalities had killed.  Before she died, Marci had shown David kindness, brought him water and played with him for a little while.  So David trusted her and therefore entrusted her with 'Morrie'.  Marci was then left to play with the doll in a room locked away from the other Legion personalities. 
 
It wasn't until Karma arrived inside David's mind that the other personalities grew restless.  Marci passed 'Morrie' to Karma, who used it to communicate with the other New Mutants back in the physical world.  Unfortunately Karma was outnumbered.  As such the Legion of personalities stole 'Morrie' and took it in turns to possess David Haller's body and fight the New Mutants; their intention was to kill Dani Moonstar. 
 
However, Illyana Rasputin realised that Karma's physical body was a gateway into David Haller's mind.  Upon her arrival in the mind she executed the Jack Wayne personality and rejoined both Karma & Marci.  The other personalities were too scared of being killed by Illyana that they passed 'Morrie' back to them. 
 
When they finally found the original personality of David Haller he explained that the doll wasn't called 'Morrie'.  The name had been a mispronunciation by Marci.  The doll's real name was Moira.  The one item that guaranteed control over Legion's body was named after Moira MacTaggert, the one woman who had cared for David Haller for many years.