Satoru Umezawa

Satoru Umezawa's triggered ability triggers whenever you activate a ninjustsu ability, and it will resolve before that ability resolves. The card with ninjustsu will still be in your hand while resolving the ability and must remain revealed until the ninjutsu ability resolves or is otherwise removed from the stack.

Puff of smoke not included.

As you activate a ninjutsu ability, you reveal the Ninja card in your hand and return the attacking creature. The Ninja card stays revealed and isn't put onto the battlefield until the ability resolves. If it leaves your hand before then, it won't enter the battlefield at all.

Once you activate a ninjutsu ability, no player can affect that ability by removing Satoru Umezawa from the battlefield. Even if Satoru leaves the battlefield before it resolves, you'll still be able to put the creature onto the battlefield tapped and attacking.

Although the Ninja is attacking, it was never declared as an attacking creature (for purposes of abilities that trigger whenever a creature attacks, for example).

The creature with ninjutsu enters the battlefield attacking the same player or planeswalker that the returned creature was attacking. This is a rule specific to ninjutsu; in other cases, when a creature is put onto the battlefield attacking, that creature's controller chooses which player or planeswalker it's attacking.