The Leader Robot is the leader of the robots (cybermen) in the Minecraft ROBOT INVASION co-op map! video.
HomelessGoomba and ImmortalKyodai meet the robot outside of their house with diamonds.
He tells them they need to come with them, otherwise his army will destroy their house filled with diamonds.
Charlie helped Homeless and Immortal escape by teleporting them away, however the Leader and the robots caught up with them and demanded that they and the "traitor" surrender. However at the last minute Charlie activated the teleporter again, this time leading to the volcano where they met Dex.
It is unknown whether Homeless & Immortal stopped the invasion or not, although judging by the lack of invading robots in later videos presumably they succeeded.