Ash and Pikachu are certainly well paired, but there's no way that Ash is going to make it through all of his Pokemon championships and Gym badges without a little more help... without a lot of help, actually. Ash has a number of Pokemon at his side helping with the hundreds of battles he's experienced. All are very important to Ash. But as a Trainer, Ash can only carry six Pokemon at a time. The six he carries determine the course of the battle.
Brave and eager, just like Ash, Oshawott snuck out of Professor Juniper's lab to follow him on his Unova journey. Oshawott is always ready to battle. In fact, when it simply hears the word battle, it let's itself out of its Poke Ball. But if the opponent looks too tough, it'll glady push Pikachu to take its place.
However Oshawott is training hard so its strength will match its enthusiasm.
It was such a strong fighter, it took Ash and Froakie a few tries to catch wild Fletching. During Ash's first Sky Battle, Ash's Flying-type friend evolved into Fletchinder. Then, when the Legendary Pokemon Moltres mistook Ash and his pals for troublemaking Team Rocket, it evolved into Talonflame to protect its beloved Trainer.
Caterpie is the first Pokemon that Ash ever catches, but he catches it in a fashion that is not standard with Pokemon Trainers - he doesn't weaken it first!
The Gligar that Ash comes across is a slow one-slower than the rest of its flock. After Ash's rival, Paul, captures Gliscor, the leader of the flock, Gligar decides to go with Ash.
Bulbasaur is a reluctant addition to Ash's arsenal. It agrees to go if Ash beats it in a battle, which he does.
Acquired in a trade with Dawn, Buizel wanted to battle instead of competing in Contests.
Which of these 6 pokemon is your favorite?
To be continued
(Source: Pokemon Encyclopedia)