A suicide note has been recovered from the boy's bag, which hints at their relationship, police said.
Doctors attending to her at Safdarjung Hospital said Roshni is in the Intensive Care Unit, and has suffered four deep injuries on the skull. Blood vessels supplying the brain have been damaged.
She has a major brain clot and her right wrist was badly slashed in the attack, a doctor said.
Police said that the family members of Akash, the 23-year-old student of university's School of Languages, who attacked Roshni, have reached Delhi from Bihar.
After the attack, Akash committed suicide by slitting his throat and also drinking a poisonous substance.
A police officer said a four-page note written in Hindi and English by Akash had been recovered.
"We have recovered a four-page suicide note from Akash, were he has written that whatever happened between both of them (Roshni and Akash) is known to their friends. So we will question friends of both of them to know what triggered him (Akash) to indulge in such a brutal attack," the officer said.
The incident has shocked students and faculty at the JNU. The campus is known as the hub of radical thinking.