
Read a spoiler before about his illness and they said it's leukemia. In the last pages of chapter 50, it shows that he's suffering from a nosebleed and has some bruises on his left forearm (shown as he was leaning on the table). Checked online and they are symptoms of leukemia.
Apprently, it is the reason why he's tough on training Falma because he's aware he has limited time left. He wants to train Falma as much as possible so that he can be ready as the replacement heir for their family.

SPOILERS
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I found pdf copies of volumes 1 to 4. I think it was machine translated with some edits so it wasn't the best translation but it was understandable enough over all.
From what I understood in volume 4, Palle went into remission thanks to Falma's intensive care and god arts. He lost all his hair though due to the treatments and Blanche cut off her long hair to make a wig for Palle. He now has golden locks instead of silver. That part made me cry a bit.
Since he's in remission and is still recovering from the treatments, Palle can only do limited activities. Falma needed to be beside/near him at all times because his holy sanctuary prevents anything from infecting Palle especially with his weakened immune system. So, very enthusiastically, Palle and Falma later wrote a medical book together. Falma's medical knowledge--specifically the medical terms he used--were too modern and otherworldly for his current world to understand. They worked together by discussing and brainstorming with Palle mostly translating unfamiliar terms to their common language. They worked really well together since Palle was medically trained already.

I wonder is he's trans? There was that glimpse of a picture and it seemed like he dressed up as a woman before. Maybe a relationship gone wrong because of that.

TRUE. That could also be his sister. I did think of that, too. Personally, the reason why I'm leaning more onto the trans backstory theory is mostly due to the director's monologue that stated " no hiding, no self-deprecation, no fear of others" when he was thinking about Shizuma being open about his relationship with Minato. And it just so happened that we got a glimpse of a photo of a woman that looked like the director during that specific line and then we're shown that the director's apparently a man. Also, when talking to Shizuma earlier, the director mentioned how Shizuma has supportive friends and family, and he's easily accepted by people. The way he was saying it and the look on his face seemed to indicate a troubled past with his family and friends. Not being shunned and not being accepted as trans COULD be the reason that consequently led to his current personality because if it was really him on that photo then the things I mentioned probably led to a change. He seemed like a bright person before

I know this is fiction but I love how they seem such a realistic couple as compared to other BL couples I've read lately. The progress of their relationship, their personalities, attitudes, how they grew to learn more...everything just seemed like it can happen in real life. Obviously, this is just my opinion and I could be way off. This thought of mine is mostly based on my comparison with other BL works anyway. I've been binge-reading BL mangas/manhwas/manhuas for about 3 days now and this is the only one I got this "realistic" feel so far. ヾ(❀╹◡╹)ノ~

Try this one too! http://www.mangago.zone/read-manga/bokura_no_tsuzuki/
I suddenly had this mental image of Silvia looking so cool as the head of the Sabnock family in the future. I mean, Sabro and all the misfits have already been hinted to be the future 13 so if Sabro focuses on his duty as one of the 13 then Silvia can take over as the Sabnock family head. Not all of the 13 members are family heads, right??? And with her creativity, I can just imagine her creating the greatest weapons ever (and so much more). She's just so cool that I can't help creating this image of her!!!