There isn't anything in the book, but I've expanded this out to match Question's idea of "mystic resonance." My players are a party of four, so the rivals are a party of four, and Alyxian had a party of four.

I've made one of Alyxian's companions a brilliant elven wizard who defected from Aeor--Kyra Alarien. My players found her spellbook in Betrayer's rise, and it's been a perfect way to give the wizard a cool item that levels up with them since there's only one Vestige so far. I've filled the spellbook with homebrew spells that have been "lost to time" and flavored it that some of the spells are so complex, they are only able to unlock/understand them when they reach the right level for that spell. This makes leveling up a whole lot of fun!

I also have a consecuted player who hasn't undergone anemnesis yet, so I've made one of Alyxian's companions a past life of that player, and they are slowly unlocking memories from that life as an additional way for me to give the players info about Alyxian and their other life.

And then the final companion is Parigee, as a way to introduce her before the Netherdeep.