They met at a Pokemon trading card competition in 2012.
The then 13-year-old student became infatuated with Xavier Chua Cheng Hao, 22, and expressed her feelings for him. He rejected her, citing their nine-year age gap.
In one conversation, she told him about some sexually graphic material she had read in a novel. Chua then told her to call him if she wanted to experience "anything of a sexual nature".
They later began talking about having sex together. The girl, who had turned 14, agreed to perform oral sex on him though they were not in a relationship. She thought he would change his mind if she did so.
Yesterday, Chua, now 25 and unemployed, was given 11 months' jail after admitting to two counts of sexual penetration of a minor in May 2013, and sometime in January and February 2014.
Eleven other charges, including one of sexual exploitation of another minor - the victim's schoolmate - were also considered.
Deputy Public Prosecutor Kelly Ho said Chua came to know the girl in late 2012 in a regional trading card competition. Their sexual activities lasted from May 2013 until October 2014, when she gave up hopes of a relationship with him.
Their sexual encounters took place at his old house in Sunrise Way near Yio Chu Kang, the public poolside toilet of Cabana Condominium in Seletar, where his family had moved to, and a stairwell of a multi-storey carpark in Hougang.
"Throughout the period when they were engaging in sexual activities, the accused knew that V1 (the victim) had feelings for him, but never considered entering into a relationship with her as he did not have feelings for her," said DPP Ho.
Chua was caught when the parents of the other girl, then 15, found him at their home in May last year. A naked photo of him was found on her phone and she had sent him photos of herself in various states of undress.
Chua's lawyer Amolat Singh said his client represented Singapore in the World Pokemon Championship in Boston last year. His client is remorseful and determined to change, he added.
Seeking a sentence of 10 to 12 months' jail, DPP Ho said Chua had no qualms about taking advantage of the victim's love for him by repeatedly asking her for sex. The prosecutor said he had taken "systematic and calculated steps" and did not commit the offences on impulse.
The maximum punishment for the offence is 10 years' jail and a fine.