Ryuuko uses Seki Tekko, the glove with a wristband to pierce her skin and draw blood that Senketsu needs to transform and give her power. That glove was given to Ryuuko by Aikuro Mikisugi (the teach in pilot shades) in episode 2.
It is colored red and is exactly what Ryuuko needs to make use of Senketsu effectively. Was it just lying around in the teacher's closet for punishing bad students and just happened to fit, or did Mikisugi make it specifically for Ryuuko? Where does this glove come from?