Blue Oak

Blue Oak (Green Okido in Japan) is the rival character in the Generation I games and FireRed and LeafGreen. He is the grandson of Professor Oak and the rival of Red or Leaf.

Appearance
Blue Oak is a slender boy with light skin, honey-brown eyes, and tall and honey-brown spiky hair.

Generation I / Generation II / Let’s Go Pikachu and Eevee
He wears a long purple shirt, dark gray pants, a yin and yang necklace, and brown boots with black straps encircling them.

FireRed & LeafGreen
He wears a short-sleeved black shirt and purple cargo pants and black shoes. He also carries a light blue pack around his waist, and has purple bands around his wrists.

HeartGold & SoulSilver
He is slightly taller and more mature-looking. He wears a black leather jacket, a black belt around his waist, khaki cargo pants, and black shoes.

Sun and Moon
He wears a black short-sleeved shirt with buttons, green pants and shoes. He also wears a pair of sunglasses and carries a gray backpack.

Personality
In Generation I, he is mostly an arrogant, cocky, rude and unfriendly rival who can't tolerate being defeated. When he bids farewell, he says this in a disrespectful manner, often saying "Smell ya later!" as he walks away. His personality is not really changed in FireRed & LeafGreen.

However, in HeartGold & SoulSilver, he became more mature and became a much kinder person.

Generation I / FireRed and LeafGreen
Blue is the grandson of Professor Oak and the younger brother of Daisy. When the player (Red or Leaf) chooses his\her starter, Blue will choose the starter who is strong on what the player had chosen earlier (Charmander if the player chose Bulbasaur, Squirtle if the player chose Charmander and Bulbasaur if the player chose Squirtle). After choosing their Pokémon, Blue will challenger the player to a battle. If he wins, he will say "Yeah! Am I great or what?", and if he loses, he will say "WHAT? Unbelievable! I picked the wrong Pokémon!".

After encountering the player several times, Blue defeats the Elite Four and becomes the champion, but is defeated by the player. When Professor Oak shows up, he tells Blue that the reason why he lost is because he forgot to treat his Pokémon with love and care. After this, Blue can be found in the Indigo Plateau, where he will fight against the player after they have defeated the Elite Four.

Generation II / HeartGold and SoulSilver
Three years later, Blue is revealed to be the new Gym Leader of Viridian City, replacing Giovanni. When the player (Ethan, Kris or Lyra) first meets him in Cinnabar Island, he mentions to that three years ago, he was the champion of the Indigo Plateau and that his reign was short-lived as he lost to Red. After that, he can be challenged at the Viridian Gym, and if he is defeated, he will give the player the Earth Badge.

Black 2 & White 2
He appears in the Pokémon World Tournament as Kanto's Champion and where he is matched up against the player to battle in the Champions Tournament.

Sun and Moon
He can be found at the Battle Tree, where the player can fight against him and Red.

Let's Go Pikachu and Let's Go Eevee
Blue appears various times in the game, but your first encounter with him is after defeating Brock. You encounter him in Silph Co in which you battle him. He is in professor oak’s lab to give you the ability to mega evolve. He can be battled on Viridian City Gym after defeating the champion the first time, once per day.

Main Article: Gary Oak
Blue has a counterpart in the anime named Gary Oak (Shigeru Okido in Japan), who is the rival of Ash Ketchum (Red's counterpart). Gary and Blue have a lot of similarities between each other: both Blue and Gary have a grandfather named Professor Oak and who is the professor, both Blue and Gary were rude and cocky rivals who always insulted their respective rivals (Red \ Leaf for Blue and Ash for Gary) at any opportunity, and both were defeated by their rivals in their final battle.

Gary makes his debut in "Pokémon - I Choose You!" where he is seen with a group of cheerleaders who cheer for him. Gary and Ash become rivals, but they do not fight each other until "The Ties That Bind". In that episode, it was revealed that Gary chose a Squirtle and that he already evolved it into a Blastoise. After Gary was defeated by Ash, he retires from being a trainer and starts to work as a Pokémon researcher.

Gary is also seen in the Diamond & Pearl series, where he traveled to Sinnoh, and now he currently works for Professor Rowan.

In the manga
Blue has a counterpart in the manga who shares his name. Like his game counterpart, Blue in the manga also has a grandfather named Professor Oak who is the Pokémon Professor and an older sister named Daisy. He is the rival of Red.

Trivia

 * Blue is the only character that has been a rival, champion, and Gym Leader.
 * Blue is the only Gym Leader who:
 * Shares his name with his Gym's Badge, as the Earth Badge is called the "Green Badge" in Japanese, matching Blue's Japanese name.
 * Uses a full party of six Pokémon in a Gym battle.
 * Does not have a specialty type or a confirmed slogan.
 * In Generation I and FireRed and LeafGreen, the player can name Blue. This makes him the first rival to have this trait, as he is followed by Silver in Generation II and HeartGold and SoulSilver, Barry in Diamond and Pearl and Platinum and Hugh in Black 2 & White 2.
 * Speaking about Hugh, the player can name him, but his default name is Hugh.
 * Blue changed his appearance six times throughout the Core Series. His original appearance was in Red, Green and Blue, and then he changed in appearance in Yellow, FireRed and LeafGreen, HeartGold and SoulSilver, Sun and Moon and in Let’s Go Pikachu and Eevee. This makes him the rival who changed his appearance the most times in the core series.