Original captors and protectors of the Titan, these Sirens have existed almost since the creation of the sea itself. They were the first to follow Tethys in recognizing the importance of the Titan’s survival and that its imprisonment would be the only way to ensure it.
Heartstone is effective on Guard of the Unnamed because its job is to block hits on for its teammates. It gives Guard of the Unnamed the ability to survive the early turns of later fights, before its Incant ability is able to give it enough armor to survive.
Runestone is effective on Guard of the Unnamed because its job is to block hits for its teammates, and Runestone helps with this. An additional reason is that Guard of the Unnamed already has an Incant ability, and you will want to put your Incant abilities on the same floor to allow you to best benefit from each of them.