To get this done you need three things I feel. At least for a Crusader Japan without an earlier meeting
1) A 'Kristian' Kyushu that refuses to persecute Christians, and starts breaking away from the authority of Kyoto and Tokyo. Probably paired with Spain. This needs to have some sort of accord, which leads to Kyushu being the first/main region in Japan that Christianity is left alone. It is messy, but having Kyushu as a whole being an internationalist Japan, leads to a whole bunch of opportunities. Primarily to do with trade and growth - which increases population, yada yada.
2) The more pious Japanese Christians learn about the Crusades, and about the Ottomans, and make the offer to Spain that if the Spanish push for a war with the Ottomans, and allow the Japanese access to Spanish ports, they'll send a volunteer army to invade Egypt.
3) Next time the Ottomans and Spanish/HRE go to war, Spain accepts the help from the Japanese. If defensive you get a late Japanese invasion of Egypt (tell that story to your grandkids), whilst if on the offence, you could get a Samurai invasion of Egypt alongside all the European action. Talk about a weird old war.
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If you want them involved in OTL Crusades? You're going to have a hard time. BUT lets try something. Forgive me, but there are inevitably a number of butterfly nets to get this to work, but I posit that most of the changes will happen in the Indian Ocean.
1) During the Roman Empire before Justinian, an Emperor authorises a Roman Trading Company that is largely owned by the Emperor. It is allowed to have a private army, but not to base it outside of a single town on the Red Sea. This basically creates an early Roman East Indian Company - with interesting results. Lets have it be successful, and becomign a leader in Roman Blue-Water Ship-building. It takes Ceylon and holds up there, converting the locals on the urging of the Emperor and various Patriarchs. Now that it has a source of valuable spices to sell to the Empire, it starts to focus even more on owning the supply of these goods. It makes attempts in Indonesia, the Philippines, etc, and succeeds in making contact with Japan. It introduces Christianity centuries earlier. If we're early enough, it could change the Asuka period to being one involving Christianity, Buddhism and Shinto.
2) Over time, this company wanes and ebbs, and it doesn't change the Arab invasion, but the Roman Trade Company loses its Imperial backers, leading to a truly independent Roman-descended Corporate State. It competes fiercely with Muslim traders as a result. (Woo, trader wars). If it is banned from Muslim ports, it focuses on controlling trade in the Indian Ocean, otherwise, continues trading through the Caliphate.
3) If the company has been fair with Japan, it has some Japanese clients and members. It makes sense that some would be so zealotic as to want to Crusade. This leads to the establishment in Ceylon of the Bushido-Templars (or something like that), essentially a place that Christian Ronin can go to have a Grand Master.
4) When the Crusades kick off, there is obviously news that makes it to the Company and the Bushido-Templars. The Company may be getting blocked out of ports for being Christian, or fear it - and the Bushido Templars have a goal now - and unprovoked, due to a combination of pro-Roman sympathies, and Christian loyalties, fight their own Crusade. Arriving a bit later than the European Crusades, and attacking Egypt from the South East. - Causing the Fatamid Caliphate to divide its forces - making the European Crusade more successful, and if the Company-Japanese Crusade is successful, leading to parts of Egypt being under Company authority.
5) Assuming a successful Company Crusade, the Bushido Templar order can set up in Eastern Egypt - creating the odd situation where a number of small, demographically limited group of Japanese and European Zealots control the Levant and Egypt, desirous of immigration and conversion. The Japanese benefit from travelling Ronin who get free passage on Company Ships, and Europe gets access to the Companies trade network.
I'd want to push a WHOLE load more detail in to justify half of the above, but I think you need something almost that level of "wtf?" if you want Japanese Crusaders. It would require Centuries of Christianity in the East.
But yeah, my PoD is an Emperor authorises a private army for a trade company

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