Sentinel

Sentinels are small flying creatures that act as guards in Tubba Blubba's Castle. In Paper Mario, Mario cannot fight them, and they cannot hurt him. When Mario gets too close to a Sentinel, it starts to flash and tries to grab him. Lady Bow can use her Outta Sight move to turn her and Mario transparent to confuse the Sentinel. If one catches Mario, it will throw him and his party out of the castle, after which his active party member will remark on how to avoid getting caught:
 * Goombario: "Mario...If they catch you, they'll carry you out of the castle. Don't you think you can hide from them by using one of our party member's abilities?"
 * Kooper: "Can't we do something so these things don't chuck us out of the castle? Doesn't anyone in our party have an ability that'll help us avoid them?"
 * Bombette: "Mario, they'll throw us out of the castle if they catch us. Can we use a party member's ability so we don't get caught?"
 * Parakarry: "This is not good. We'll be thrown out of the castle when they catch us. Can't we get around these creatures by using the ability of one of our party members?"
 * Lady Bow: "These things will try to catch us and throw us out. Use my ability to turn invisible and we'll fool them."

When Tubba Blubba chases Mario and crashes the bridges above the main room, the Sentinels are crushed by the debris. They disappear after the chapter is over.

Sentinels are similar to Gnips from Super Paper Mario.