Hi Crossmr, I understand why you would be skeptical.

Civil defense is annoying, at least in my opinion. That statement was a personal opinion of mine.

Entering a country with the passport of your country of citizenship is not illegal. In cases where you hold multiple citizenships, as long as you are identifying yourself with a valid passport from any of those countries you have the right to do so.

Now as for the renouncing citizenship bit: According to Republic of Korea International Law Section 15 (I forget the specific clause number, sorry) In instances where a Korean citizen (which the OP is since he was 2 when he moved) obtains a foreign citizenship, if within 6 months time he does not show any intent to retain such citizenship (this is really broadly written on purpose and can mean anything from filing out drivers license application to getting a bank account, etc) they lose their Korean citizenship. Which means that, if the OP didn't do anything intentionally to maintain a presence in Korea, the Korean government automatically assumes that he gives up the citizenship. Since he probably didn't (he was a kid after all) even without filing any paperwork his citizenship status is already considered "dead". It's effectively renounced.

Now I say effectively, because he didn't actively seek to get rid of citizenship. Which is why i say "dead". For all the government knows, the OP is a person who's whereabouts or identity is uncertain. In such cases the 주민등록번호 as well as his family register and his official records are flagged and rendered inactive. This done so that if someone who claims to be him (which is the perspective the government would view it from) they are notified immediately and can make appropriate actions. And more importantly, in situations of inheritance, etc, the record itself would not be expunged and still be recoverable.

Now to the last point you raised, if the embassy (he probably shouldn't even go to the embassy imho) or some other government agency (probably customs and immigrations) finds out who he is.... they will just shrug and say "so what?" Its seriousy a non-issue. He's not done, as you say. As a Korean citizen it is totally not illegal to get a korean visa stamp in your own passport. It's just never done because it's not needed. However, he has duel citizenship and it's up to him to decide which nationality he will claim as he enters another country. If he claims his Canadian citizenship (which again, is within his rights) he must get a visa stamp. At no point is he doing anything illegal or lying.

Now as for he fear of getting drafted, it's mandatory in Korea and eventually the OP will have to serve or the equivalent. But in cases where the person does not have permanent residence within Korea, extensions are granted and special arrangements will be made within the military for his eventual conscription. But that's in the future. If there are no outstanding warrants or conscription notices the the OP at the time of his visit to korea, no one in Korea will care. Unless he's some celebrity or related to some jaebol.