Pikachu has been with Ash since the very beginning. Since Ash was late, he didn’t get his choice of the three starter Pokémon. So he wound up with a Pikachu. Pikachu and Ash are really tight now, but this wasn’t always so. In that first episode, Pikachu wasn’t exactly friendly. When the duo faced an entire flock of Spearow, Ash fought to save his Pikachu and their first connection was made.
Watch how their friendship started in the first episode: