I will try to give a cohesive response with what I know :) Keep in mind that what I say may not be 100% accurate, as I don't speak Japanese nor have read Shadow Cry (SC).

I will credit this tumblr post + link to a Chapter 1 translation & review of the novel by someone who has actually read Shadow Cry. Most of my summary is taken from these + additional facts were cross-examined from various machine translated JP posts.

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Shadow Cry is a 2007 novel focused on Strega. The novel also has Shinjiro, Akihiko, Ken, and an original character named Izumi. It switches perspectives between the characters but is relatively centered on contrasting Jin and Izumi’s mindsets. It takes place in the summer of the game (which adds to why it's hard to view as canon but I digress).

Izumi was a child in the Strega experiment. He acted as an older brother to Jin and protected him as children, resulting in a scar on Izumi’s eye. Strega thought Izumi died during an exploration in Tartarus, but he survived and was taken in by Mrs. Amada. Though he now lives with a different woman and her children, Izumi wanted to find Mrs. Amada’s killer because he knew Ken was searching for them. Izumi decides to send a request through Strega’s website, unaware that Strega were his old friends. Unrelated to the request, Izumi and Strega reunite by coincidence during the dark hour. Jin is thrilled, Takaya is intrigued, and Chidori doesn’t like Izumi cause she barely remembers him. However, Izumi has his own family now, and there is a gap in mindset between Izumi and Jin towards attachments.

Jin is envious of & respects Izumi's positive outlook towards family. However, the novel concludes with Jin affirmed in his mindset he should live for Takaya. By the end of the novel, Izumi becomes this out-of-control shadow consuming monster that Strega has to kill. I think Jin ultimately does it with Izumi’s own knife. (Izumi became a monster because his uniquely weak nameless persona eventually became part of him, not exactly a shadow (?).)

I am not sure if Jin kills Izumi to protect Takaya & Chidori, but I’ve seen depictions of it. This could mean 1. Jin realized he cares for Takaya like how Izumi cares for his family, or 2. Having to kill Izumi made Jin realize once again that all attachments are meaningless, as he just killed an old friend. My hypothesis is it’s a combination.

The novel portrays Jin's inner conflict in additional “caring” ways. Takaya doesn’t like to clean, so he cleans. The furniture in the Strega hideout was moved by Jin & Takaya during the dark hour. When Chidori needs hot water, Jin assists (but only after being threatened). I think she orders him around a few times actually. I know at some point Strega eats ramen, and I’ve seen various art imply that Jin was the one who cooked or bought it. Also, it’s canon that Jin plays with his grenades in public but I’ve seen implications that Jin entertains children with it. lol. Furthermore, all of Strega babysits/ visits the child Izumi lives with named Saya.

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Besides Strega & Izumi, Shinjiro seems to be important to understand Jin. In all other P3 media, Jin & Shinji are very hostile towards each other. SC portrays them with a “mutual respect” towards another. A specific line sums it along the lines of “Jin doesn’t hate people like Aragaki.”

While Jin is investigating Mrs. Amada’s killer, the monster (Izumi) attacks him. Shinjiro, who was in the alley paying his respects, saves Jin. At Takaya’s (?) suggestion, Jin takes Shinjiro out for ramen as repayment (but also maybe see if Shinjiro was Mrs. Amada’s killer?).

I don’t know what they discuss over ramen. The rest of this paragraph is hypothetical, and may just be completely fan made. I’ve only seen some artists depict it as Shinjiro telling Jin he should determine what life personally means to him, and not just rely on Takaya’s meaning. (Again, this is the point of the novel.)

Based on the P3 manga, Jin sees a parallel between himself and Shinji because they’re both doomed and meant to live alone. But I think Jin’s respect for him in SC may come from how they’re both responsible for other’s wellbeing. I’ve seen art depicting Shinjiro and Jin hanging out on the back-alley stairs either 1. discussing this similarity, or 2. “busting each other’s chops”; but that’s probably all fan-content. Overall, I think their potential dynamic is interesting and wish it was in the game.

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Other trivia I’ve taken from the novel is that all of Strega may have some markings due to their artificial personas. They’re translated as “birthmarks” but described more as “scars”. Chidori has two parallel vertical lines on her back. Saya points this out, saying that princesses are actually angels who have to remove their wings (?) according to her dad. Takaya’s tatoos cover his scars. I don’t know if Jin is addressed, but I’ve seen headcanons of burns on his neck or arms.

Chidori comes off like a little sister in SC due to her demanding nature. She also doesn’t like Izumi because she finds him flirty (sound familar as to how she treated Junpei). But she is nice to Saya. Not much I’ve seen about Takaya, but his depictions are as chill as ever; This matches the new scenes of him in Reload. But he is also a little playful.

A lot of Jin fan work (including the P3P anthology "Strega Days") depicts him as having a more “childish” view towards girls. I think this aligns with some of the things he says in the 1st chapter. I’m not sure how to explain this, but for clarification I would not call it misogynistic.

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Strega definitely seem more rounded in SC than their one-track mindsets in canon. Any other information on Shinji, Ken, or Akihiko you will have to read the linked review for more information. That's the only place I've been able to find further on them.

After a couple hours of typing this, I think it's cumulative enough. Hope this was interesting and you learned something new :)