(It is canon to the Voltron series on Minimax TV programme. It's a little different tho.Keith is Zarkon's son,but didn't tell anyone.So,theoretically,Keith is one of the Galra's,but isn't bad like they are.

One day,while Keith was walking on a random planet,in his galra form,the paladins found him and captured him by the orders of Allura.

He was put in the black lion,where Shiro could watch over him while the paladins were going back towards the castleship.

(You're Keith.)