Ace was gobsmacked when he heard the system's explanation about this mysterious voice's owner, and he had to admit the chances of this information being true were pretty darn high.
Furthermore, the system was clearly telling him not to mess with her or let her enter his body, or if she did something, even the system wouldn't be able to do anything to prevent it.
As for the Soul Resurrection Bead, he got it when he tricked Winter into joining the Thief's House. There was a soul hidden in her using this Soul Resurrection Bead, but the system got rid of it when she joined the Thief House.
But Ace didn't know what it was used for or what its grade was since the system only told him its name while its grade and description were filled with question marks (???), so he almost forgot about it.