I've seen a lot of people suggesting Mydei/Castorice, Phainon/Castorice, and even Mydei/Castorice/Phainon, but for all the fandom's shipping (and everyone should feel free to ship what they love; your ship is valid, fam!), I actually kind of think that...

Castorice is a bit of a Mydei/Phainon shipper herself.

Although Castorice is of course just a good person who is doing what she can to help Okhema, she also is quick to assist Phainon specifically to save Mydei, quick to try to keep Phainon calm because that's what will help him get to Mydei quicker, and she just brings Mydei up out of the blue to Phainon several times throughout the story.

It's Castorice who halts Phainon's ascension ceremony to ask where Mydei is, because she expected him to be there for Phainon.

It seems to be a given for Castorice that if Phainon needs him, in Phainon's most important hour, Mydei should obviously be with him. She knew Mydei would come.

Even before that, when Phainon was feeling down, Castorice admits she doesn't know how to comfort Phainon herself, and instead... brings up Mydei to comfort Phainon???

Girl thought Quick, how can I raise Phainon's self-esteem? and Mydei's nickname for Phainon was the first thing that seems to have come to mind. 😂

She really said "You're not lame, Phainon; Mydei thinks you're a hero!"

Okay, being more serious--even putting shipping aside entirely, it's just overall clear that Castorice perceives the close comradeship between Phainon and Mydei (probably moreso than Phainon himself) and understands how important having that close friendship is to Phainon, who seems to have nothing else left outside of the Chrysos Heirs at all.

She seems to be able to tell how much Phainon needs people in his life who believe in and can stand beside him, and seems to have clocked that Mydei is definitely one such person. The game tells us players clearly that Castorice is an incredibly perceptive person who is sensitive to the feelings of others, and part of that includes her continuing to verbally recognize, throughout 3.0, the support Phainon gains from his close connection to Mydei.

I think this is just another cool touch--but also maybe another subtle nod from the devs. Castorice won't even let Phainon have a single scene where Mydei isn't mentioned lol.

And finally, one last crumb based on a pet theory...

"As I've Written"

We don't yet know who is responsible for actually writing the character profiles in the "As I've Written" book--although the rewards section is called "Author's Recompense" and the player get rewards for "composing sagas," alongside the interact button being "Write Story," there's actually an entire achievement teasing the fact that the Trailblazer doesn't know who actually wrote the book:

It's not remotely written in a style the Trailblazer would write in, and it also contains information the Trailblazer (at least currently) has no way of knowing at all, like the details of Castorice's backstory.

At this point, the real author could be anyone. But I feel like there's a few things pointing in favor of the idea that the real author might be Phainon. It could also be Anaxa or Cyrene or even Mem too, but hear me out...

First, the book's design is reminiscent of Phainon: the book features prominent sun/moon symbols, has the same blue-white-gold color palette, and even the design at the bottom of the book resembles the design along the front of Phainon's coat:

The book also seems to be strongly foreshadowing that someone is going to lose their way, step onto a dark path, or end up making a terrible mistake.

In Tribbie's chapter:

In Aglaea's:

And of course in Phainon's chapter, where the foreshadowing is strongest:

If this "one who has lost their direction" and "lost themself," the "flawed hero," are all references to Phainon, then the book over and over again seems to be--for the player--foreshadowing Phainon's downfall. Or, from the other perspective: This is a record written by someone who has witnessed (or experienced) the downfall and knows what is coming.

There's also the fact that while Phainon's chapter is written in third person, the narrator occasionally slips in some hints that they know what's going on in Phainon's mind:

And there's also this moment from Mydei's chapter:

We already know that this is not how Mydei behaves around people he doesn't know. When Mydei isn't familiar with a person, he doesn't banter with them--he doesn't even bother with them. He barely speaks directly to the Trailblazer the entire 3.0 plot, for example! He doesn't remotely seem like the type of person to sit down at a table and drink with someone he doesn't know.

We also know that he's already scolded Phainon several times for trying to act like an expert in Kremnoan legends:

(Thank Streetwise Rhapsody from Youtube for these screencaps because I forgot to screencap it myself lol.)

And the icing on the cake:

The exact phrase "amateur historian" again.

To me, this all but confirms that the "true" author of the "As I've Written" chronicle is probably Phainon, which finally brings me to the actual Phaidei crumb I wanted to discuss all along:

Mydei's story is listed as chapter 10 of the book. Yet for some reason --even though we get the book only after completing nearly the entire 3.0 questline, when the player has definitely met Aglaea, Castorice, and Tribbie already--Mydei's story comes first.

While Castorice, Tribbie, Phainon, and Aglaea all share the same memory crystal, Mydei has his own separate memory crystal, not shared with any other character, and it is given to the player first, before anyone else:

Phainon really said "My man is more important than the rest of us combined."

Tribbie is chapter 1. Phainon is chapter 7. Aglaea is chapter 9. But for some reason, we jumped all three of those characters to present chapter 10 first. Theoretically you could say that it's because we went to Castrum Kremnos and fought Nikador? But, story-wise, was Mydei the most important? The Trailblazer met Phainon and Tribbie first, got to actually play Aglaea for a sequence of this story, and traveled alone with Castorice. Mydei is the character the Trailblazer actually had the least connection to in the whole 3.0 storyline, so it doesn't seem that the story is truly what determined the order characters' chapters were given to us.

At the end of the day, in a book that seems it could be written by Phainon (from the future? the past?), Mydei was given special treatment and came before anyone else.