Blue, Red, and Green are all featured in Pokemon Let’s Go, but Blue is by far the easiest one to find on your Pokemon Journey.
As soon as beating the Pewter Gym, Blue will be hot on your tail throughout the rest of the story to help and antagonize your trainer. His full relevance isn’t until after earning the seventh gym badge in Cinnabar Island however.
After leaving Cinnabar Island, head straight back to Professor Oak’s lab where Blue will give you the Mega Stones for all three of the Kanto Starters, Venusaurite, Charizardite X and Y, and Blastoiseinite.
After finishing the main story, you can find Blue heading the Viridian City gym, where you can battle Blue to test your strength once a day.