I made my World History/Myth nerd friend (who's never seen ROR) guess the Einherjar by their designs! (Her explanations for each in post-body)
Here are her explanations for each choice!
Lu Bu - Very sure he's Lu Bu, those plumes are an iconic signature of his. He's got a topknot (Chinese male hairstyle) and his beast-face buckle is a classic Chinese armour motif, marking him as a military man. Tattoos are normally associated with outlaws in Han Chinese culture so he's probably an outsider (also Water Margin heroes were inked). No armour means he doesn't need protection and his face is very scarred and bored, so he's not noble. So, Lu Bu is the closest for me, Xiang Yu was also seen as a strong, unbeatable brute but he is normally shown wearing armour.
Adam - The naked man covered only by a leaf is very classic Renaissance Adam, and his skin tone and looks are also of that time, he wouldn't look like this if they portrayed him more accurately geographically, but it's an iconic look.
Sasaki Kojiro - His sword being drawn from over his shoulder means it's too long to wear at the hip, Kojiro's own sword was unusually long. Popular novels and interpretations of Kojiro have him in red, as he is nicknamed the "flashy dandy in red" with a long sword. Only thing stopping me from saying it's him 100% is the fact he looks so old, it could be a sword saint I can't recall who was said to live up to being an old man.
Jack the Ripper - Very clearly Victorian-inspired. At first, I was leaning Wells due to all the gears and clockwork everywhere giving me "time traveller". But Jack is more popular and has been fictionalized as a gentlemanly killer before, for a Japanese manga, I can see a popular figure like Jack being this character instead.
Raiden Tameemon - Looks like a grappler, I want to say he's a famous sumo wrestler but having another Japanese character only 2 matches apart from the last one makes me unsure. If not, he could be a Greek one, I'm not too well-versed in wrestlers so I can't say who for either.
Buddha - Thought he was like a beach bum or something at first lol. But, he's definitely Buddha. The ūrṇā, his long earlobes, the hair piled in curls on top of his head, he's got an orange sash like a monk with beads hanging at his waist. If the Buddhist symbols weren't so prevalent, I'd guess someone else as he looks too laidbacked for the Enlightened One.
Qin Shi Huang - Definitely Chinese royalty. He has a mandarin-collared top, his robe is draped like a cape, his brocade with dragon-like patterns and his upturned boots are all Chinese court dress, not to mention his posture is giving "I give the orders here". His claw-like nail guards were worn by nobility to show they never did manual labour, and his colours of gold and that yellow were reserved for Emperors alone. His blindfold could be a reference to a special crown emperors wore with strings of jade beads hanging in front of their face to veil their gazes, so subjects can't read their expressions. At first, I was leaning Kangxi because of the nail guards and yellow fit being a Qing-dynasty apparent thing. Wu of Han is also a great conqueror... but for a tournament of all of humanity, the First Emperor makes the most sense to me.
Nikola Tesla - Had to double check I couldn't say Tony Stark lol. Has to be an inventor of some kind, Tesla is the most famous but without the moustache he looks very uncanny, but my gut is telling me Tesla over Edison. Archimedes could be a fun pick as an ancient inventor who gets modern technology, and Faraday was the father of electromagnetism and did a lot of work with coils but feels like too niche a pick over the others in his niche.
Leonidas - Almost went with Patton when I first laid eyes on him. But, on closer look, his steel shield is similar to an aspis, a central Greek warfare shield of the hoplites. Again, his age threw me off but out of my three choices, Leonidas is the one I think is most likely to show such age.
Soji Okita - I researched the Shinsengumi because I saw them in Gintama lol. Okita was young and cheerful so he's a clear choice here... even though his "counterpart" in Gintama is nothing like him.
Simo Hayha - The Finnish flag, a sniper, need I say more?
Sakata Kintoki - Was a bit hesitant to guess a folk hero because of the others but the giant axe, all the gold colours on him and the Japanese clothing lead me to Kintaro (even though he has red skin in myth). Pretty sure he's a demon-human mix so he would fight for the humans. Could be Momotaro since I'm pretty sure Momotaro is more known in Japan but I see no peaches. (Also four potential Japanese fighters??)
Nostradamus - The clothing is very Renaissance Europe. The sleeves, the fitted vest over loose shirt, the puffy breeches, stocking and flat shoes are all around mid-to-late 1500s fashion, around France and Italy. Very anime design, was leaning to feminine da Vinci but all the plant symbols like the leaf on his chest and shoes, and his scarf is like a vine or seed pod, which means he's tied to herbs and botany. So, I'm leaning to Nostradamus who was also an apothecary. If there were more engineering hints, I'd be more willing to say da Vinci.
Rasputin - Very hard to tell from the picture given lol. Going da Vinci, if the last one is Nostradamus, having da Vinci feels like a must here, plus the long flowing beard and hair is very Da Vinci. Rasputin and Moses are fun alternatives I thought of.