He is a very cocky and somewhat unfriendly young man, typically bidding people farewell with "Smell ya later!" (Japanese: バイビー！Baibii!), and his primary motivation throughout the game is to become the greatest Trainer. Aside from that main goal, he also seems to take satisfaction from always being one step ahead of the player character. He eventually accomplishes his goal, defeating the KantoElite Four and becoming the Pokémon Champion, but is quickly defeated by the player, losing the position.
In Pokémon Red, Green, and Blue, Pokémon Yellow, and Pokémon FireRed and LeafGreen, Blue is the player's rival. He used to be a good childhood friend of Red but became mean as they grew older. He believes that Red is his rival because they are of the same age and height. He will meet up with the player as they journey across the Kanto region, challenging them every once in a while to test their skills. Like the player, he receives a starter Pokémon from Professor Oak, his grandfather: the one he chooses will be of the type which weakens the type of the player's choice. In Yellow, he will take the Eevee Professor Oak had intended for the player.
Eventually, Blue becomes the Champion at Indigo Plateau, though he is defeated by the player before Professor Oak arrives to congratulate him. Once Professor Oak arrives, he deems Blue to have not realized how to care for Pokémon, which he commends the player for. Frustrated, Blue journeys to the Sevii Islands at Professor Oak's request, obtaining new Pokémon to use in his matches against the player at the Indigo Plateau (as all in-game Champions return to their position on the player's return to their hometown).
In Pokémon Gold, Silver, and Crystal and Pokémon HeartGold and SoulSilver, however, it is revealed that, after he was defeated at the Indigo Plateau by Red (Leaf does not appear and is not mentioned in the remake games, and did not exist at the time of Generation II), Blue returned to Pallet Town. As Viridian Gym's Leader, Giovanni, had abandoned it on his defeat at the hands of Red, Blue took the position, now handing out the Earth Badge to Trainers who bested him. In Generation IV, he also distributes TM92 (Trick Room) to those who win the Earth Badge, but does not give a TM out in Generation II. He has not lost his nature as a traveling Trainer, however, and frequently leaves his Gym, creating problems for Trainers who wish to challenge him. When they do find him wherever he is, he points them in the direction of Viridian Gym for an official league battle. Blue will not battle the player until they have the other seven Kanto Badges in the remakes. However, in the originals he can be challenged as early as after the meeting on Cinnabar Island.
In HeartGold and SoulSilver only, the player can face Blue in a rematch at the Fighting Dojo. The method of obtaining his number is slightly different from the other Gym Leaders. The player must have their Pokémon at maximum friendship and have it massaged by Daisy Oak seven times. After the seventh time, she will give the player Blue's phone number when spoken to. He can be called for a rematch on Sunday night.
While he does not appear in X and Y, Blue is mentioned by a woman in the Magenta Plaza Pokémon Center in Lumiose City, who says that Blue traveled to the Kalos region to study abroad.
In Pokémon Sun, Moon, Ultra Sun, and Ultra Moon, Blue appears at Alola's Battle Tree, having been invited to be a leader of the facility alongside Red. When the player first enters the facility area, Red and Blue will appear to greet the player and congratulate them on becoming the Champion of Alola, after which the player gets to battle against one of them.
In Pokémon: Let's Go, Pikachu! and Let's Go, Eevee!, Blue is first encountered in Pewter City after the player has defeated Brock. He mentions how he also left on a journey from Pallet Town sometime earlier than the player, and, as Professor Oak hadn't made a Pokédex by the time he left for his journey, had had to resort to scribbling down on a map what Pokémon he encountered and where. He's later briefly met again aboard the S.S. Anne, where he will give the player a Shalour Sable and warn them and Trace about Team Rocket. If talked to aboard the ship, Trace can reveal that Blue actually defeated the Elite Four several years ago. When Team Rocket later takes over Silph Co., Blue is met again at the first floor of the company's office building in Saffron City. He talks about going to the Pokémon League to ask the Elite Four's help in taking down Team Rocket, but wishes to battle the player and Trace first before doing so. After the player has defeated Blue, Trace battles him next while the player starts climbing the building.
After the player has earned seven Badges, Blue appears in Professor Oak's Laboratory in Pallet Town, where he congratulates the player and Trace from defeating Team Rocket at Silph Co., rewarding them with a Key Stone each and also giving the player all the Kanto starter Pokémon Mega Stones. Once the player has defeated Giovanni at the Viridian Gym, Blue meets the player in front of it and reveals that he had been asked multiple times to take over the Gym, and now that Giovanni has left it, he decides to finally accept the request and become the new Viridian Gym Leader. During the post-game, the player can rematch Blue at the Gym once per day.