Sylveon is one of the Eeveelutions available for capture in Pokémon Legends: Arceus, and it has the chance to appear in Space-Time Distortions.
Pikachu is one of the staple Pokemon of the Pokemon series. Check out this guide to find out where to find Pikachu in Pokemon Let's Go Pikachu & Eevee. This way you can have a Pikachu at your side if you went with the Eevee version.