As soon as you arrive in Vermilion City, talk to the police officer with the Squirtle nearby. If you’ve caught at least 60 Pokémon (not unique species; it includes duplicates), she’ll give you a free Squirtle at level 16.
The Pokémon Center is directly to the west of her. Heal up your Pokémon if you’d like. There’s a Hiker on the western couch that will trade you an Alolan Geodude for any of your normal Geodude. You can make this trade an unlimited number of times and each Alolan Geodude will be at level 16, regardless of the level of your traded Geodude. Alolan Geodude is a Rock/Electric-type, giving it some unique advantages.
South of the Pokémon Center, you’ll find a Poké Mart to the east, which has the usual selection of items at this point in time.
|X Sp. Atk||350|
|X Sp. Def||350|
The person on the bench to the west of the Poké Mart — a Black Belt in Let’s Go, Pikachu and a Beauty in Let’s Go, Eevee — will ask whether you prefer Growlithe or Meowth.
Whichever you answer, they’ll challenge you to catch 5 of the Pokémon exclusive to your version — 5 Growlithe in Pikachu or 5 Meowth in Eevee. You can find Growlithe back on Routes 5 and 6, and Meowth on Routes 24 and 25 north of Cerulean City.
After you’ve caught those Pokémon, return to that person on the bench and they’ll reward you with a Persian if you’re playing Let’s Go, Pikachu or an Arcanine if you’re playing Let’s Go, Eevee, both at level 16 and both otherwise unavailable in your current version.
When these Pokémon are outside of their Poké Ball, instead of following you, you’ll get to ride on them and they both move very fast compared to your standard walking speed.
Inside the fancy building to the west of the Persian/Arcanine person is the Pokémon Fan Club. Talk to the chairman on the big red couch and he’ll tell you a story about his beloved Rapidash. After a fair amount of dialogue, he’ll reward you with either the Eevee Set or the Pikachu Set, with the set given matching the version you’re playing. These are a full set of clothing items for your player, although your partner Pokémon doesn’t get a set — it doesn’t need one, as it’s already a Pikachu or an Eevee!
If your friendship is high enough with your Pikachu or Eevee, you can return to the chairman and he’ll give you the Raichu Set in Let’s Go, Pikachu or the Eeveelution Sets in Let’s Go, Eevee.
The Raichu Set is available for both your character and your partner Pikachu and allow you both to dress up kind of like a Raichu. The Eeveelution Sets, on the other hand, are eight separate sets for both you and your partner Eevee that are each styled like one of the evolutions of Eevee, so there’s a Flareon Set, a Jolteon Set, and so on.
You can raise your friendship with Pikachu or Eevee by playing with it and by feeding it berries.
Head east of the Poké Mart and you’ll find a branching path. You can head south to the S.S. Anne, which is where you need to go to proceed with the game’s story, or you can head to the east to check out Route 11 and a little bit of the Diglett’s Cave.
As you head south to the S.S. Anne, you’ll flash the Sailor the S.S. Ticket you have, and he’ll give you a Sailor Set for being the 100th guest on the S.S. Anne. This is another set of clothing for both you and your partner Pokémon, resembling a Sailor.
Keep heading south and you’ll arrive onto the S.S. Anne.