In HM DS, you can marry Leia the Mermaid. In HM Cute DS, you cannot marry her, but you nonetheless can experience all four of her Heart Events (as well as those of other 'Magical Girls' in both games).
In order to experience Leia's 4th and Final Heart Event, aka the Yellow Heart Event, you must be aware of these requirements:
1. You must have experienced Leia's first 3 Heart Events and have Leia at Yellow Heart Level;
2. You must have a Mystrile Fishing Rod or Fishing Rod of a higher level than Mystrile;
3. You must catch the Bottled Message using any Fishing Rod at Mystrile Level which means, at least in the original HM DS, that you cannot have a Fishing Rod lower than Mystrile and, if it is higher than Mystrile, only charge it to Mystrile Level. (I think in Cute, you may be able to catch it with higher levels of Rod with a full Blessed or Mythic charge but I would have to look at my game journals to make certain).
4. YOU ONLY CAN CATCH THE BOTTLED MESSAGE IN SPRING.
In HM DS, you then had to go to Daryl's house with the Bottle in your hands to trigger the 4th Heart Event. In Cute, you can go there with the Bottled Message in your rucksack and it still will trigger. Either way, once the Event is experienced, Leia will return to the sea.
She will visit the Beach of Forget-Me-Not Valley after that each week. If you wish to marry her in HM DS and did not have her at full red heart level before her return to the sea, you can ply her with Big Fish then until she reaches red heart level.