Love Letter Blox Fruits: The Son Quest

Love Letters are three love letters to collect for Robotmega, as part of The Son Quest which leads to the second version of the Dark Blade. Each one is hidden behind a destructible wall or door in the First Sea. The Fandom warns from the start - the community considers this puzzle to be useless and purely decorative.

How to get it

The three letters are found at Frozen Village, in the Skylands, and at Marine Fortress, behind obstacles to destroy.

Where are the three letters

Location	Where to look
Frozen Village	Behind a wall that you must destroy with a destructive move, for example the [Z] of the Dark Blade
Skylands	Inside a house, behind a door to break the same way
Marine Fortress	Behind a wall - once it's destroyed, you fall into a maze made of other destructible walls

How to start the collection

The starting point is Dog House (Indra) - by interacting with him and choosing the right dialogue options, he asks you to gather the three letters for Robotmega as an apology.
The source adds right after that the puzzle "can be started by talking to Robotmega". It thus gives two entry points without specifying which takes priority - start with Dog House (Indra), and go see Robotmega if nothing triggers.

Good to know

These letters belong to The Son Quest, the chain that leads to Dark Blade v2.
The Fandom clarifies however that this puzzle is considered useless by players, and that it's more decorative than anything else. In other words, don't count on it to unlock anything on your own.
Beware of the homonym - there's another item called Love Letter, the one from the Dog House event, which has nothing to do with this one.
You need a move capable of breaking the scenery to reach the three letters - without it, none are accessible.