Supergirl's secret identity is a closely held secret known only to Superman, her foster parents, and the Legion of Super-Heroes, of which she is a member for a time. Like all Kryptonians, Supergirl is vulnerable to kryptonite. Streaky the Supercat, her orange cat, acquires temporary superpowers as a result of its exposure to "X-kryptonite," a form of kryptonite Supergirl accidentally created in an unsuccessful attempt to neutralize the effects of green kryptonite. Comet the Superhorse, a former centaur, is Supergirl's equine companion.