"This character is essential to the Plot" is something that happens in story driven CRPGs.
Gods abowe, under and everywhere in between ... how many times i have to repeat it?
She can leave your party ...
She dont have to be invited in your party ...
She can be permanently killed at any point ...
Therefore she - is - not - essential - to - the - plot.
The item she posesses is ...
Shadowheart: around 170cm tall, humanoid, talking, alive ... carrier of McGuffin, not important on her own.
A box: around 20cm in diameter, polihedric in shape, not talking, presumabely also somehow alive ... essential for the plot.
See the difference?