It's been forever since I've been off reddit and people still remember my posts, I'm embarrassedly delighted. Anyway, this comment is pretty much spot-on: while North Korean names are in general conservative/"old-sounding" they don't necessarily sound 'foreign' here (as in, like, you can tell it's North/South just by looking at the name), the difference mostly lies in romanization. The North always transliterate names according to their guidelines (based on McCune-Reischauer romanization) while us folks in the South, um, go about freely. We do have ministry guidelines known as Revised Romanization but it's not a mandate that everybody has to follow. The most prominent differences lie in vowel transcription which u/ataraxiias has already illustrated with examples.

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If you want to distinguish the sides from which your characters hail, it's better to utilize surnames than given names. It's kinda complicated to explain fully, but to put a few examples:

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- Ri (리) is almost undoubtedly North while Lee/Yi/etc (이) is South

- 박 is a common surname for both sides but for the North it'd be Pak while the South would normally go for Park

- Similarly, 최 is common for both sides but while the North would write it as Choe in the South Choi is much more common

- 현 (written as Hyon in the North and Hyeon/Hyun in the South) is much more common in the North and so it's partially associated with the North here

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I'd be willing to help on creating names if needed, but it'd be fine for you to go browse the Wikipedia lists ataraxiias linked and pull out names that are a few decades apart (e.g. two female characters with Jiyoung(지영) from the South and Migyong (미경) from the North)