If this is the one I'm thinking about, it's at the top left area, where you can rematch Grim. There's an opening behind where Grim stands where you can find Dina.
Damn, how could I miss that opening gap lmao? But anyways, thanks for pointing me out.
Now I just finished DLC story. Truly didn't expect a good story out of eroge lol. And is it just me or the DLC ending indeed kinda break 4th wall? Since you need play around with save and load game file in order to defeat DLC's last boss, and Tseng even mentioned about the "outside" power that even gods (in-game gods, I suppose) cannot match. Basically with every save created, we created a new alternative world/reality/timeline or whatever, and we technically won by deleting Tseng's existence from a save file where a happy ending is. But idk, somehow I feel kinda dissatisfied with that type of ending imo... I mean after the whole world's Cell connected to Alcy's, but still lose in a way. I still prefer the true ending from the base game, but that's just me saying.
Still a good game regardless... wonder if there's any similar game like this, where the ecchi and the story is balanced.