Shigeru is a character who is roughly the same age as Satoshi (though Shigeru is assumed to be slightly older, due to the height difference between the two); they were playmates and next-door neighbors when they were younger, but drifted apart when they both discovered their love of Pokémon, according to the video game. At the start of the series his rivalry with Satoshi becomes a recurring motif. Like the video games, manga and anime, Shigeru is described as an overconfident and arrogant trainer who's knowledge of Pokemon stems from being the grandson of Professor Okido. As Satoshi's rival, he always seems to be a few steps ahead of Satoshi, much to Satoshi's chagrin.

Prior to his introduction, Shigeru usually comes off as an arrogant show-off in the beginning. However, his personality begins to change as he matures. Later on in the series, he grows into a nicer and more humble person in both the anime and the video games. Throughout most of the series, he and Satoshi have an ongoing rivalry through most of the Kanto and Johto League sagas. However, this would soon diminish at later point. After his crushing defeat at the Indigo Plateau Pokemon League, his attitude towards Ash, who finished in the top sixteen far ahead of his rival, changes significantly after this turning point. His dealings with Pokemon seem to mature, as he starts travelling alone, wanting to better reestablish himself as a trainer.

His rivalry with Satoshi comes to a head at the Johto Leaue Silver Conference semi-finals, both trainers giving it their all, with Satoshi narrowly coming out the victor. After his lost, he then decides to no longer compete for the same goal as Satoshi, but instead dedicate his career into Pokemon research. Although no longer Satoshi's rival, he does tend to motivate Satoshi on ways of bettering himself as a trainer, at times scolding Satoshi when becoming overconfident over his wins particularly in the Orange League and Battle Frontier. In a sense, Shigeru is what urges Satoshi to start yet another new journey by traveling to the different regions across the world, such as Johto and Sinnoh. Despite his many absences in the series, Shigeru continues to remain a vital character, and one of Satoshi's most valued friends. He is bound to make more appearances during the Diamond and Pearl Saga, since he seems to have decided to be both a Pokémon trainer and a Pokémon researcher there. He guest-appears in episode forty-five of the Diamond and Pearl series helping to preserve a group of highly endangered Shieldon with Professor Rowan and his team.

(Source: Wikipedia)