Having Curie as a companion means making good or neutral choices during your travels, while avoiding bad or selfish choices.
Later, the player can transfer Curie's mind into a female Synth body as part of a side quest (which we'll cover in a moment). Curie is a French-accented Miss Nanny robot repurposed for conducting research in Vault 81 where you can recruit her after the 'Hole in the Wall' quest that opens the Vault.
