It's not because that's her power or supertition or anything else, it depends on the age of the person who sees her.
She looks like a grandmother to kids (up to17 years old..?) and women who are pregnant, and she looks like a woman to adults.
Sungjae saw her as a grandmother when he was still a student and she appeared as a young woman in his eyes when he turned into an adult.
That's why that girl she met in the hospital greeted her "Hello, grandma!".
And she appeared as a grandmother in Goeun's mother's eyes because she was pregnant at that time..
[+214][-6]
1. [+79][-2] This might sound random but I find her really pretty..ㅠㅠ I'm so jealous of her because she looks really good in red..
2. [+77][-1] I heard that she's not as powerful as God but she's more powerful that Grim Reaper and Goblin.. That's why they can't win over her..
3. [+29][-0] I think she appears as grandmother to kids who are underage because they're more innocent than the adults..?
4. [+5][-0] Grim Reaper doesn't recognize her, though..?
5. [+4][-0] Nope.. Euntak's mother still sees her as grandmother even after she gave birth to Euntak..
6. [+3][-6] Isn't this too forced..?
7. [+2][-0] Euntak's mother still sees her as grandmother even after she gave birth to Euntak.. I don't think your theory is right..
8. [+1][-0] I think she still appears as grandmother to those who have kids..? And she just appears as young woman to those who don't have kids..
9. [+1][-0] That grandmother make up looks disgusting to me..
10. [+1][-5] But she still appears as a young woman when Grim Reaper first met Sunny..