Grand Oak gives the player their choice of the following starter Pokémon when they start the mobile version of Pokémon HOME. While this is not mentioned by Grand Oak, each of them has their respective Hidden Ability.
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"Excellent! Access successful! Now, can you see me? ...Ah, it seems this is working! My name is Grand Oak. I'm a renowned Pokémon researcher! It's a pleasure to make your acquaintance. I'm the one who developed Pokémon HOME, which—as I'm sure you're aware—you're accessing right now. What a grand day to remember! oh, but look at me getting carried away. I haven't even given you a chance to introduce yourself! Please, tell me your name."
"Ah... So your name is <player>! You see, my dream is to create an absolutely grand Pokédex by registering all the Pokémon that exist in the world! But if I'm being honest, I don't think I'm able to do it alone... And that's where you come in! Please, you must help me! You can fill the Pokédex by bringing Pokémon here to Pokémon HOME. Of course, I will do everything in my power to assist you as well! Pokémon HOME has some grand features that will help with gathering Pokémon! First and foremost, there's the trading functions! These let you trade Pokémon with users from all over the world! Additionally, you'll also be able to receive Pokémon as Mystery Gifts! Hah, I can see you're in awe of the grand features I've developed! ...What's that? You don't have a Pokédex? Well, in that case, I think I can give you one as a special gift! Choose the Pokémon you want!"
"Are you happy with the <Pokédex category> Pokémon <Pokémon>?"
"Brilliant! If you want to know more about the features of Pokémon HOME just ask Poké Boy. Poké Boy is waiting for you in your room, along with your Pokémon! Now then, I wish you the best of luck!"