In what chapter does rukia see ichigo use his hallow mask for the first time?

Rukia first sees Ichigo use his Hollow mask in Chapter 423 of "Bleach." This moment occurs during the battle against the Espada, specifically against Grimmjow Jaegerjaquez. Ichigo dons the mask to enhance his power, showcasing his Hollow abilities in a critical fight. Rukia's reaction highlights the significance of this transformation in Ichigo's character development.