The ninjutsu ability can be activated during the declare blockers step, combat damage step, or end of combat step. If you wait until after the declare blockers step, because all combat damage is dealt at once, Kaito won't normally deal combat damage.
Kaito enters attacking the same player, planeswalker, or battle 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, planeswalker, or battle it's attacking.
If at least one creature in combat has first strike or double strike, you can activate Kaito's ninjutsu ability during the first-strike combat damage step. In that case, Kaito will deal combat damage during the regular combat damage step, even if he has first strike.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Richard Luong
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Although Kaito enters attacking when his ninjutsu ability resolves, he was never declared as an attacking creature (for purposes of abilities that trigger whenever a creature attacks, for example).
Kaito's fourth ability cares whether opponents lost life this turn, not how their life totals changed. For example, an opponent who gained 2 life and lost 1 life in the same turn still lost life that turn.
As you activate Kaito's ninjutsu ability, you reveal Kaito from your hand and return the attacking creature. Kaito isn't put onto the battlefield until the ability resolves. If Kaito leaves your hand before then, he won't enter at all.
If an opponent lost life this turn and subsequently lost the game, Kaito's fourth ability still counts that player when determining how many cards to draw.
The ninjutsu ability can be activated only after blockers have been declared. Before then, attacking creatures are neither blocked nor unblocked.
Kaito's second ability causes him to stop being a planeswalker during your turn as long as he has at least one loyalty counter on him. While he's a creature, damage dealt to him won't cause loyalty counters to be removed. You can still activate Kaito's loyalty abilities.
One or more stun counters on a permanent create a single replacement effect that stops the permanent from untapping. That effect is "If a permanent with a stun counter on it would become untapped, instead remove a stun counter from it."
