William Curl of DeKalb is charged with first-degree murder, criminal sexual assault and arson. He appeared in court Saturday in DeKalb County.
Judge James Donnelly told Curl he could face the death penalty if convicted.
Keller, of Plainfield, was last seen Oct. 14 when friends said the art student was headed to a nearby park. Two days later, burned human remains were found in the park.
In court Saturday, prosecutors said police found Keller's burned clothing and her cell phone near the remains. A public defender has been appointed for Curl. Police say he was known to frequent the park.
"It's crazy for a lot of residents because nobody expects this; nobody thought of it. I mean, the cops have been here all week but weren't telling us anything," said Curl's neighbor Chris Bushman.
Keller's family members told ABC7 they were pleased police had someone in custody.
"We're happy that the case is moving forward. We're happy that they have the person they believe did this for the sake of safety of others and ourselves," said Keller's cousin, Mary Tarling.
(The Associated Press contributed to this report.)