Yuffie, Materia Hunter

If the target of Yuffie's last ability is illegal as the ability tries to resolve, it won't resolve and none of its effects will happen. You won't attach any Equipment to Yuffie.

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.

The ninjutsu ability can be activated only after blockers have been declared. Before then, attacking creatures are neither blocked nor unblocked.

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.

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.