SHAH ALAM, Malaysia — Malaysian prosecutors on Thursday refused to dismiss the case against a Vietnamese woman accused of assassinating the half brother of North Korea’s leader, Kim Jong-un, just days after her co-defendant was set free with charges dropped.

The Vietnamese woman, Doan Thi Huong, 30, is now the only person facing trial in the killing of Mr. Kim’s brother, Kim Jong-nam, in February 2017 as he prepared to catch a flight from Kuala Lumpur International Airport.

Both defendants were accused of killing Kim Jong-nam by smearing VX nerve agent on his face in the crowded terminal. But they said they thought they were participating in a prank for a reality show and had no intention of killing Mr. Kim. Their lawyers said they were made scapegoats by North Korean agents who, working on Kim Jong-un’s behalf, recruited them and then fled the country.

On Monday, prosecutors dropped the case against Ms. Doan’s Indonesian co-defendant, Siti Aisyah, 27, after intensive lobbying by the Indonesian government, including a personal appeal from President Joko Widodo to Malaysia’s prime minister, Mahathir Mohamad.