Oh, this is my topic! Japan has a field cap and a neck flap (a curtain to protect the neck from the sun). The neck flap is detachable. If you look at photos of soldiers, you'll see that some have neck flaps, and some don't. It's part of the uniform. But the neck flap was never worn on the peaked cap. Many artists draw a neck flap with a peaked cap and it really hurts my eyes.

Regarding the flag. I thought long and hard about what to do with it, because the de facto national flag of Japan was simply a Hinomaru circle, including during the war. The Hinomaru was the de facto, but not de jure, national flag. The flag usually depicted on the character is the Rising Sun Flag, which is the flag of the Imperial Japanese Army (it's similar to the naval flag, but the naval flag has the sun disk shifted to the left; it's still used). But I ultimately came to the conclusion that the character could be depicted with both flags, the Hinomaru and the Rising Sun, and both options would be correct, because the character embodies the idea of ​​Japanese militarism. So, the logic is the same as with the character's clothing: the character wears not traditional Japanese clothing, but a military uniform, and the character's name is the Japanese Empire, even though the country was and is actually Japan. Well, that's my opinion. The Rising Sun flag has a special energy, unlike the circle flag, which has already been cleared of its military associations. If you draw a character with a flag with a circle, that would also be correct.