Of the main characters, only Goku has learned to use Kaioken (in Dragon Ball Z, in the anime). After SSJ was introduced in ...

From the time Goku first became a Super Saiyan in the Namek Saga to the arrival of the sagas such as Kaioken Form and, more recently, God Form and Ultra Instinct, each of the forms has contributed ...

The Kaio-Ken doesn't always get the respect it deserves in DBZ, but it's arguably a cooler transformation than Super Saiyan with even better fights.