Withdraw (からにこもる Withdraw to the Shell) is Squirtle's side special move. Squirtle retreats inside its shell and speeds across the stage, powered by a jet of water shooting out the back of its shell. During this attack, Squirtle's shell behaves like a Green Shell for a limited amount of time. If another player jumps on Squirtle, the attack ends and Squirtle becomes stuck on its back for a few seconds. Squirtle cannot take damage during Withdraw.
It is important, especially for beginners, to note that Withdraw is not a rolling attack like Yoshi's Egg Roll or Jigglypuff's Rollout; Squirtle is unable to change direction on its own or charge the attack beforehand. Spinning up and down hills can alter the speed in which the withdrawn Squirtle will move. The attack can be cancelled if the control stick is moved at the opposite side that Squirtle is attacking. The invincibility provided by the move is sufficient to block many Final Smashes that do not involve grabs. Marth's Critical Hit can be negated, but not Ike's Great Aether.
Like some other moves, Withdraw's speed can be altered by smashing the control stick, instead of just tilting it. Also if Squirtle gets footstooled by another player while he is performing withdraw in the air, then the move will be canceled and Squirtle will be put into a helpless state of free fall and be unable to recover until he lands on the ground or grabs a ledge.
 Description from the Instruction Booklet
Tuck into its shell and slide around the stage. Cancel by tilting the Control Stick in the opposite direction of the slide.
Withdraw is a Water-type non-damaging move introduced in Generation I from the Pokémon series. The move is only capable of being learned by Pokémon with shells. It has no effect outside of battle. In battle, the move increases the user's defense by one stage. However, as Withdraw is not an offensive move, Withdraw's usage in Brawl may be based on Squirtle's use of Rapid Spin in the Pokémon anime. Squirtle learns Withdraw at level 28 in Generation I and at level 10 in Generation II and onwards.