Yuffie, Materia Hunter

Yuffie 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.

You can't use Yuffie's last ability to try to attach an Equipment to Yuffie if that Equipment can't legally be attached to Yuffie.

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, Yuffie won't normally deal combat damage.

Although Yuffie enters attacking when its ninjutsu ability resolves, it was never declared as an attacking creature (for purposes of abilities that trigger whenever a creature attacks, for example).

If you lose control of Yuffie or it leaves the battlefield before its last ability resolves, you won't gain control of the target noncreature artifact.

As you activate Yuffie's ninjutsu ability, you reveal Yuffie from your hand and return the attacking creature. Yuffie isn't put onto the battlefield until the ability resolves. If Yuffie leaves your hand before then, it won't enter at all.