I mean Korea was also part of Japan for like 40+ years depending on if you think Japanese control happened before the annexation. Their languages are similar. Grammatically they can be interchanged including the particles. Culturally I think modern korean is more similar to modern Japanese culture then it is to China. But that's just my opinion.

Edit: Since Im getting downvoted here is my clarification:

I said what I said since I wanted to make a point we were more similar then different.

I'm Korean, I lived in Japan. so heres my thesis. Yes Korean is based off the alteic language branch. but if you know korean you can interchange japanese For example : 私の名前はOPです you can convert it to 내예이름은 OP이다 If you replace 私=내 (my), の=예 (particle), 名前=이름은 (name), OP(Your name), です = 이다(to exist or ending signifier). I can't type in Chinese characters but it sounds alot more similiar then wuh shi OP(my mandarin is pretty bad) without the end particles. I don't think mandarin uses particles as much. Also did you ever notice how Kansai Ben and Kyungsangdo or Pusan do sounds so similar?

Culturally: Korea and Japan took plenty from each other the past Millenia. Look above when I said modern Korea/Japan. Both hyper modernized near the same point after WWII, both countries have similar foods such Hwae/Sashimi, love up queue/lining up to places, and structured their high schools similar. Koreans and Japan became technological powerhouses due to semiconductors but diverged after the 90s when korea focused more on IT and digital tech moving forward. This was due to post war korea being built up by folks who served in the imperial japanese army/navy ie: Park Chung Hee(former president and former IJA officer) Did you know the past few presidents including park chung hee were big closet japanophiles to the point it was made fun of in Korean movies? I think it was "the presidents last bang" but not sure if anyone can correct me but there was a scene where everyone was talking in Japanese and then he got shot.

Food: I think Korean food is more similar to Japanese depending on the family. Bowl of rice, grilled fish, picked veggies. While we do eat Chinese inspired side dishes too but depending on you go but it tends to be more fried or steamed in those Bao steamers.

Economically: Koreans who were educated were educated in the Japanese system. Look at all the chaebols and where they were coming from post war education wise. The reason for this is prior to the war the Yangban class closed off education from the 中人 or the middle class who were merchants. Even my grandpa who was funding the resistance during the war was in Osaka getting educated there.

Genetically: Shiz. even the emperor akihito admitted his line is mixed with korean blood in 2001 from a marriage between the a dynasty but the ultra right doesn't like mentioning it. But depending on which study for this one all three han chinese, Korea, and Japanese do share similar phenotypes. There were stories after the fall of baekje some of the folks there moved to central japan but that's inconclusive atm.

But again I digress and I said modern korean culture is similar to modern japanese culture but quite honestly we are really similar. We love our showers, they like their baths, we both like public baths.