Leanne Zaloumis, who is charged with trespassing on Cowell's west London property, spoke only to confirm her name and address - Brownhill Road, Catford.
She was remanded in custody at West London Magistrates' Court for her case to be heard by a crown court judge.
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The man, in his thirties, was found with stab injuries on the upper deck of a bus in Handsworth, Birmingham, on Monday night.
Thousands of passengers now face delays, with Southeastern trains warning "all services" would be affected.
Paramedics attended a scene in Willesden, north-west London, on Monday afternoon where they found the teenager with stab wounds.