The echo and Hydaelyn's contact

[Cilia]

Hydaelyn is constantly broadcasting her "Hear, Feel, Think" message; the Echo is just what lets people pick up on it. Cause and effect were mixed up for the longest time because people believed Hydaelyn speaking to them meant she had chosen them for a special destiny or whatever - that simply isn't the case.

Granted, it's still not clear how the Echo prevents tempering, or if it does so at all in light of Emet-Selch's admission that all the Ascians were tempered by Zodiark when they summoned him. My best guess is still that we are tempered by Hydaelyn, but owing to her raison d'etre she doesn't compel service or influence our behavior in any way.

[Rosenstrauch]

Granted, it's still not clear how the Echo prevents tempering, or if it does so at all in light of Emet-Selch's admission that all the Ascians were tempered by Zodiark when they summoned him. My best guess is still that we are tempered by Hydaelyn, but owing to her raison d'etre she doesn't compel service or influence our behavior in any way.

One thing that muddies the water even further is that both Fordola and Zenos possess the Echo through artificial means. Fordola uses it to stop Lakshmi's attempts to temper the NPCs alongside Arenvald and WoL during a solo duty, and Zenos uses it to not only resist Shinryu's attempt to temper him, but is able to subvert the primal's will and take control of it himself. Unless both of them are tempered by Hydaelyn, too, it does seem like the Echo provides resistance to tempering on its own. It might just be that it relies on the strength of the person's will—the Convocation weren't strong enough to resist Zodiark, whereas Fordola and Zenos were strong enough to resist Lakshmi and Shinryu respectively.

[Cilia]

One thing that muddies the water even further is that both Fordola and Zenos possess the Echo through artificial means. Fordola uses it to stop Lakshmi's attempts to temper the NPCs alongside Arenvald and WoL during a solo duty, and Zenos uses it to not only resist Shinryu's attempt to temper him, but is able to subvert the primal's will and take control of it himself. Unless both of them are tempered by Hydaelyn, too, it does seem like the Echo provides resistance to tempering on its own. It might just be that it relies on the strength of the person's will—the Convocation weren't strong enough to resist Zodiark, whereas Fordola and Zenos were strong enough to resist Lakshmi and Shinryu respectively.

Making things even more unclear is that, when your Echo first protects you from being tempered by Ifrit, he says something along the lines of "Your soul has already been claimed by another. And mortal souls cannot hold two blessings." In other words he's not unable to temper you because of the Echo, but because you're already tempered, implicitly by Hydaelyn.

Of course, I am also of the mind that the Echo does provide innate protection by allowing a contest of wills, but given contemporary Source souls are so much weaker and the power of the Ancients so fragmented in non-Echo users they can't win that contest. If one does have the Echo (an Ancient soul), their will is empowered enough to resist it - but even primals on the scale of true deities, like Hydaelyn and Zodiark, by Emet-Selch's admission cannot be resisted.

It's also important to remember that the Blessing of Light and the Echo are two separate things; the Blessing is what power Hydaelyn lends us, while the Echo is the awakened power of an Ancient's (in our case Azem's) soul.

It's rather unclear. Pretty sure the writers didn't have an endgame in mind when coming up with this, lol.

EDIT

I may be crazy here, but I thought that the Starshower and cutscene with Lahabrea was quite literally the first thing that happens after making a new character, while you're asleep on the carriage/boat, and Hydaelyn only speaks to you after you get your first crystal.

I started my first toon recently, so it's fresh in my mind:

You do have a vision of Hydaelyn and Lahabrea in the intro cutscene, while sleeping on your transport to [Eorzean City-State]. There is no starshower; that comes after getting the Water Crystal from the Lv5 instanced battle where you first meet your city-state's Scion warden.

You have another starshower vision after getting all six and obtaining the full Blessing of Light, for some reason.