A Pokémon’s nature decides what stat increases and losses a Pokémon will naturally receive, excluding the HP stat. For example, him companies Nature will increase a Pokemon’s Attack stat at the cost of lowering the Special Attack stat. Knowing this, we can take a look at Piplup’s stat pools and its evolutions and see which nature would be ideal in Pokemon Legends: Arceus.
Since Piplup, Prinplup, and Empoleon basically have the same stat pools, and each stat increases at about the same rate with each evolution, it will be easy to pick a good Nature to fight with this lane.
The highest stat in the line is Special Attack, with Speed being its weakest point. Knowing this, we can decide what Nature we want. Since the Special Defense stat is so close to matching the Special Attack stat, and the entire line doesn’t need to make much use of the regular Attack stat, we can see Calm Nature as a potential winner here. This will give us that boost to Special Defense to match the Special Attack stat we were looking for, and makes it worth the Attack stat reduction.
An alternative to Calm The nature, especially if you want to pump that special attack stat as high as possible, would be to look at the Modesto Nature. This will give us the same losses as the Calm Nature: An affordable hit to the Attack stat, while also boosting Special Attack to be head and shoulders above the rest of the stat pool.
In general, this makes the Calm Nature better if you focus on a Special Defense based build for your eventual Empoleon, or, if you want to lean more towards the Special Attack stat, you can choose the Modesto Nature to best suit your needs.