Stormblood Yotsuyu/Tsuyu storyline is so good

[Cleretic]

She'd probably be tempered, and she definitely wouldn't be able to go out and about, so she'd need to be kept somewhere safe. After 5.4 (SPOILERS FOR THAT PATCH) they'd be able to cure her. Not sure what would happen after that.

What would she be tempered by? Tsukuyomi was essentially a primal born of her own rage and hatred; the EE2 points out it doesn't even resemble the actual Tsukuyomi of myth. So she's tempered by... herself? Would that just make her very stubborn?

[Anonymoose]

What would she be tempered by? Tsukuyomi was essentially a primal born of her own rage and hatred

When Elidibus is cosplaying as Zenos and sets up the false flag for Doma, he tells Asahi that incarnate-summoning (via a ritual + the infused Kojin relic triggered by her desire) will (lacking the Echo) indeed temper her. Yes, "to herself", to put it simply. But recall that a primal manifests in a form determined by its summoners' intentions, volition, state of mind, and desires - all of which can (and often do) change with new experiences, new information, or simply the passage of time.

Being "tempered to yourself" as you were only a short time ago might not amount to much of a practical difference. Being "tempered to yourself" at a moment in the distant past might lead you to become something altogether different than you otherwise would have. It's interesting to think about, at least.

Consider for comparison/contrast with Yotsuyu, perhaps... Thordan, Tiamat, or the summoners of Zodiark.

Citation Referenced

When you have found her, you will initiate a ritual to call forth an eikon. I will instruct you in the necessary steps. Yotsuyu's faith is unreliable. But as a child raised to believe in the kami, she will serve as a vessel for one of the Kojin's gods. She need only wish it to be so. The power will seem a gift, but the eikon's essence will consume her. She will be no more than a husk, a slave to whim and desire.

GERMAN
Des Weiteren werde ich dich in die Geheimnisse der Götzenbeschwörung einweihen. Sobald du Yotsuyu gefunden hast, wirst du sie als Medium benutzen. Ihr Glaube ist zwar nicht stark genug für eine Beschwörung. Aber in ihrem Land gibt es unzählige von diesen Kami. Wenn man Yotsuyu ausreichend motiviert und dazu noch ein Artefakt der Kojin einsetzt, sollte sich einer rufen lassen. Natürlich wird die Gottheit sofort von ihr Besitz ergreifen. Die Starken haben nun mal die Angewohnheit, die Schwachen zu ihren Sklaven zu machen.
I will also initiate you into the secrets of eikon summoning. As soon as you find Yotsuyu, you will use her as a medium. Her faith is not strong enough for a summoning. But there are countless kami in her country. If you motivate Yotsuyu enough and use a Kojin artifact, one should be summoned. Of course, the deity will take possession of her immediately. The strong have a habit of making the weak their slaves.

FRENCH
Avant que tu ne partes, je vais t'enseigner le rituel d'invocation secret des Primordiaux. Tu devras à ton tour le transmettre à Yotsuyu quand tu l'auras retrouvée. Elle n'a probablement aucune foi religieuse, mais étant originaire de ces contrées orientales où sont révérées des myriades de kami, elle fera parfaitement l'affaire. Il suffira d'un artefact kojin et de ses désirs pour qu'elle prenne la forme d'une divinité. Naturellement, une femme aussi veule qu'elle ne pourra pas imposer sa volonté. Une fois le Primordial invoqué, elle en deviendra la prisonnière, comme une âme inerte, dépourvue de tout libre arbitre.
Before your departure, I will teach you the secret summoning rituals regarding primals. You will have to pass it on to Yotsuyu when you find her. She probably has no religious faith, but being from these eastern lands where myriads of kami are revered, she will do perfectly for the job. All it will take is a Kojin artifact and her desires for her to take the form of a divinity. Naturally, a woman as weak as she is will not be able to impose her will. Once the primal is summoned, she will become its prisoner, like an inert soul, devoid of any free will.

JAPANESE
これより貴様には、神降ろしの秘儀を授ける。ヨツユを見つけたならば、その手法を手ほどきせよ。奴には、信仰心など露ほどもなかろうが、八百万の神が息づく東方の生まれならば、依り代にはなる。紅甲羅の神器と成すべき願いがあらば、神のひとつも降ろせよう。むろん才なき女は、自ら身に宿した蛮神の虜となり、願いに囚われた生ける亡者と、なりはてるだろうがな。
From here, I will teach you the secrets of summoning a deity. When you find Yotsuyu, teach her the method. She may not have the slightest faith, but as she was born in the East, where dwell the eight million kami, she can serve a vessel for one. A sacred treasure of the Crimson Shells will react to her desire, facilitating the summoning. Of course, lacking the faculties to control it, she become a prisoner of the eikon that resides within her; she will be as the living dead, imprisoned by her desires.

POSSIBLE PANSLATION
Prior to your departure, I will bestow unto you the secrets of eikon summoning. Once you have made contact with the acting viceroy, you are instruct her in the necessary ritual. Yotsuyu's faith is far too weak for a conventional summoning to succeed. However, given that she was raised surrounded by belief in the Yaoyorozu no Kami, she should be possessed of sufficient sentiment to trigger a reaction from a sacred treasure of the Kojin. With the proper motivation, she will serve as a vessel for the eikon it manifests. The entity will consume her mind, of course. Lacking the willpower to resist, she will from that moment be driven solely by the desire which beckoned it forth.

[Cleretic]

When Elidibus is cosplaying as Zenos and sets up the false flag for Doma, he tells Asahi that incarnate-summoning (via a ritual + the infused Kojin relic triggered by her desire) will (lacking the Echo) indeed temper her. Yes, "to herself", to put it simply. But recall that a primal manifests in a form determined by its summoners' intentions, volition, state of mind, and desires - all of which can (and often do) change with new experiences, new information, or simply the passage of time.

Being "tempered to yourself" as you were only a short time ago might not amount to much of a practical difference. Being "tempered to yourself" at a moment in the distant past might lead you to become something altogether different than you otherwise would have. It's interesting to think about, at least.

Consider for comparison/contrast with Yotsuyu, perhaps... Thordan, Tiamat, or the summoners of Zodiark.

This is true, and Elidibus himself probably brings the clearest example, albeit in a bit of a sideways way. (Spoiler-ing the rest because I think the OP might still be in post-Stormblood.)

I've described current Elidibus as 'a self-portrait of a self-portrait', basically one person's gradually distorted view of themselves over millennia of being the core/an extension of Zodiark.

He's a very extreme version, though, that's someone tempered by themselves across twelve thousand years. Yotsuyu would probably have never lived long enough to show that. ...especially because she's naturally a very stubborn person, there's a good chance you cure her tempering and she doesn't even notice.

[StormChase]

When Elidibus is cosplaying as Zenos and sets up the false flag for Doma, he tells Asahi that incarnate-summoning (via a ritual + the infused Kojin relic triggered by her desire) will (lacking the Echo) indeed temper her. Yes, "to herself", to put it simply. But recall that a primal manifests in a form determined by its summoners' intentions, volition, state of mind, and desires - all of which can (and often do) change with new experiences, new information, or simply the passage of time.

Being "tempered to yourself" as you were only a short time ago might not amount to much of a practical difference. Being "tempered to yourself" at a moment in the distant past might lead you to become something altogether different than you otherwise would have. It's interesting to think about, at least.

Consider for comparison/contrast with Yotsuyu, perhaps... Thordan, Tiamat, or the summoners of Zodiark.

(SHB AND EW SPOILERS)

If you've done the Bozja questline it's also why the ancient Gunnhildr's Blades felt so threatened when the shrine maiden they chose had the Echo. They made their plan with the assumption she'd be tempered by the transformation.

I know Emet-Selch claimed Zodiark tempered his summoners but the Loporrits said tempering was added to the primal summoning process after the Sundering. Maybe Elidibus ended up how he did because he'd already died in the initial summoning.