MUKILTEO — A man whose girlfriend was convicted last month of torturing and starving his 10-year-old adopted sister now is facing his own legal troubles in connection with the case.

The girl disclosed in December that her brother, Derron Alexis, had beaten her with a wire and a belt, according to court papers filed Monday. She also told investigators that Alexis and his then live-in girlfriend, Mary Mazalic, would taunt the hungry girl by eating in front of her.

The girl was severely malnourished when she was removed from the couple’s home in August 2011. She weighed just 51 pounds. Her body had lost nearly all its fat and her body had started metabolizing her muscles for energy. Doctors also discovered that she was covered with scars and wounds.

Mazalic remains in jail and faces decades in prison when she is sentenced next month.

Prosecutors on Monday charged Alexis with first-degree criminal mistreatment. He is accused of withholding food, medical attention and other basic necessities of life from the girl.

He has not been arrested. Alexis, 43, is expected to be arraigned next month in Snohomish County Superior Court.

Snohomish County deputy prosecutor Lisa Paul said Monday she believed it was in the best interest of the case to wait to file the charge against Alexis until after Mazalic was prosecuted.

At one point, the lawyers had contemplated calling Alexis as a witness. Midway through the trial, Alexis was assigned an attorney after concerns arose that he may want to invoke his right to remain silent so as not to possibly incriminate himself if he was later charged. Neither side ever called Alexis to the stand.

The girl testified that Alexis had beaten her often. She also said he was present when Mazalic assaulted her. The child withheld that information from investigators until December because she said she cared about her brother and he was part of her family, Paul wrote in court papers.

Prosecutors also allege that the defendant and his mother visited the girl while she was hospitalized last year. The girl’s adoptive mother reportedly told the child not to say anything bad about Alexis to the authorities, court papers said.

The woman had sent the girl to live with her son and Mazalic in the summer of 2010. The New York woman testified last month that school officials where she lived had determined that the girl needed to be placed in special education classes, but she wanted the girl enrolled in a mainstream class.

Mazalic and Alexis enrolled the girl in the fourth grade in Mukilteo without telling school officials about her special needs. Alexis reportedly told investigators that he and Mazalic both took care of the girl. He worked nights as a mechanic but often was home in the morning while the girl was getting ready for school, court papers said.

He said he was involved in disciplining the child and was familiar with her eating schedule. He also told police that the girl was well fed.

The girl told investigators that Alexis sometimes told Mazalic to feed her. He also sometimes gave her “some bad oatmeal, not really cooked,” Paul wrote in court papers.

Prosecutors also allege that Alexis failed to get the girl medical attention. Her adoptive mother sent medication with the girl when she moved to Washington. Alexis reportedly told police that when the medication ran out, instead of taking the child to a doctor, they gave her medication that had been prescribed to someone else.

Prosecutors say that there is additional evidence that Alexis lied to state officials about the time he spent in the home.

Mazalic received state disability assistance and needed a caregiver in her home. Alexis was listed as Mazalic’s full-time caregiver and reportedly told state authorities that he never left her alone.

A state welfare worker didn’t know the girl was living in the house and never met her.

“Had the worker known that a child was being left in the home with Ms. Mazalic, alone, she would not have allowed it because in her opinion Ms. Mazalic did not have the patience to be around children,” Paul wrote.

The girl was removed from the couple’s home after two store employees called Child Protective Services with concerns about the girl’s condition. Mazalic brought the girl to the plus-size clothing store to buy herself a bathing suit for a cruise. She and Alexis reportedly took the cruise while the girl was in the hospital recovering from severe malnutrition and a kidney infection caused by being forced to wear diapers.

Diana Hefley: 425-339-3463; hefley@heraldnet.com.