Twenty-two-year-old Rosendo Xo Pec was charged Wednesday with murder with special circumstances and could face the death penalty.
It's unclear whether he has an attorney.
The 76-year-old Willis recorded the 1960s songs "Someday You'll Need My Love," ''Act Naturally" and "Take My Heart."
She was living on the street when she was attacked shortly after 4 a.m. on New Year's Day.
Prosecutors say she was sleeping in a strip mall parking lot in Santa Ana when Pec began sexually assaulting her.
When she screamed for help, prosecutors say Pec repeatedly punched her in the head and choked her to stop her screams.
She died at the scene.
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