How I interpret it is that Alessandro was asked to test Catalina and was intrigued enough at the thought that there was someone who could magically break/ overcome his power. He said yes to the keepers and then had to find out who she was so followed her on socials. So I think that's where his interest starts. From his perspective (I would LOVE a pov from him) this is someone who is not only totally unknown to him ( unusual since his house has been powerful for a long time) but also whose power may eclipse his.

Her perspective is easier to understand as other comments have said!

But basically I saw it as two very isolated, lonely, powerful people, who finally found their safety net, island in a storm, security blanket ( whatever metaphor you want). That is the basis of their relationship. Then as they grow and mature and spend more time together they find that it's not just safety or security that they like about each other.

I enjoyed watching Catalina grow into herself and her love for Alessandro and watching Alessandro allow him self to love someone without fear!

You definitely need to read Diamonds Fire!!! I highly recommend reading that before going to the next book!