If you were wondering whether Monster Harvest has co-op or multiplayer, the answer is no. Monster Harvest is a single-player game through and through and the developer plans to focus on making the single-player experience exceptional.
The developer has stated that while it is a single-player game, they may consider implementing multiplayer at some point. They did not mention what factors would influence this decision, but it is likely the game's sales and reception would affect that decision.
As the game shares many similarities with Pokémon and Stardew Valley, multiplayer be indeed be added later. This was the case with Stardew Valley. The farming sim game added multiplayer at a later date so it wouldn't be surprising if the same happened with Monster Harvest. Both games can be considered wholesome which tends to do well with couples that play games co-op.
Pokémon features many multiplayer aspects such as trading and battling which allows players to share their Pokémon and strategies. A feature like this would do well in Monster Harvest as you can raise and train your Planimals like Pokémon. Being able to trade Planimals would allow players to have access to new companions much earlier in the game where their choices are limited.
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