A court was played video footage of a police interview with the teenage girl in which she described crying in pain as her boyfriend used a knife to inflict permanent injuries.
A jury at Leeds Crown Court also heard the teenager had an interest in the Holocaust, Nazism, genocide and serial killers.
The youngster is on trial with another boy accused of conspiracy to commit murder and a range of alternative charges.
The court has heard the pair, both now 15, hero-worshipped Eric Harris and Dylan Klebold - the teenagers who killed 12 students and a teacher in Colorado in 1999.
It is alleged the pair were planning an attack at their school in the North Yorkshire market town of Northallerton and shared a list of people they wanted dead.
During the interview, the girl described how she became "smitten" with the defendant after meeting him through social media.
The student described how her boyfriend would post live suicides on Instagram, as well as images of dead babies and pictures of "shooters" who had murdered people.
She said: "He said he loved having people scared of him because it would make him feel powerful."
Jurors heard the boy was spoken to by police on one occasion because of his behaviour on social media.
The girl said her boyfriend "admired" Columbine killer Eric Harris and condoned the high school killing.
She said: "He thinks it's valid because society killed them all."
She added: "He got so hooked on Eric Harris and liked how he was feeling and related to this person."
The court heard the teenager kept a diary with the words 'natural selection' on the front. She said he had also drawn swastikas in the diary.
The girl said he accessed the dark web for information on weapons and bomb making.
She described how the boy had an Instagram account closed down after he posted a livestream of himself attacking a baby doll with a penknife and covering it with fake blood.
She said the teenager had the word Satan carved into the wall above his bed and 'redrum' - murder spelt backwards - written on his door.
During the interview she described how the relationship became "toxic".
She agreed to allow him to carve his name into her back.
Describing the incident, she said: "I lifted my shirt and it felt like he was sawing with a pen knife, dragging really deep.
"I was crying and saying 'can I stop, this hurts.' He said 'no let's get this done'."
Asked by the officer how she felt after the incident, she said: "I don't know how I felt. It was kind of that numbing feeling where you don't know whether you have done a good thing or a bad thing."
She said her boyfriend later carved her name into into his own arm but spelt it wrongly.
The boy is also charged with unlawful wounding. He also denies a charge of aggravated burglary of her parents' house.
She said one night he messaged her and said he was going to visit her house and they would do a "full NBK".
The girl said she looked the term up online and discovered it stood for Natural Born Killers.
She said she became concerned as it was a film about a character who kills his girlfriend's parents before going on a killing spree.
At the end of the interview, the girl broke down in tears and said: "I loved him. I really, really did and in some ways to start with he was the first person to really make me feel worth something.
"But he is not that person, and it's so hard.
"I don't think I will get that person back."
The trial continues.