Isaac

Isaac is one of the supporting characters of The Fault in Our Stars. He had an eye cancer known as retinoblastoma, he also had one glass eye and one real eye, then the other real eye was removed and he is NEC. He and Augustus are best friends, and through him Augustus befriends Hazel. He was previously in a relationship with Monica, until she broke up with him upon hearing he would go blind. Isaac is known to like video games and has a wide sense of humor, as evidenced when Hazel visits him after he undergoes his surgery; even through the most intense pain he jokes, "Come over here so I can examine your face with my hands and see deeper into your soul than a sighted person ever could".

Appearance
In The Fault in Our Stars, Isaac is described as being a long-faced, cool, skinny guy with long blond hair swept over one eye, and having rather large, thick glasses. In the movie, the two features listed that he doesn't have is his long blonde hair and hair that swept over his glass eye.

He lost one eye to his cancer when he was younger, so he had a glass eye in place of the missing one until his remaining eye was surgically removed, leaving him NEC (No Evidence of Cancer) but blind.

Augustus
Augustus and Isaac are very close friends, in fact, they were friends before the Support Group meeting that had led to Hazel meeting Augustus. Augustus had attended the Support Group at Isaac's request to support him under the stress of going blind. It was thanks to Isaac that he and Hazel met in the first place. They hang out a lot and not much other than what is portrayed is said about their friendship - but it is evident that they're very close. Augustus seems to be Isaac's best friend even after both eyes are removed.

Hazel Grace Lancaster
Isaac enjoys Hazel's company and they befriended each other very early in the book. In the Support Group, before becoming good friends, Hazel Grace says she and Isaac communicated through small sighs. Although they may not have been friends for a long time so far, it's seen that he and Hazel are very good friends and have bonded over time. Neither of them have shown romantic feelings towards each other.

Monica
Isaac and Monica were said to have a very close relationship and were together for fourteen months. Monica later dumped him because she didn't want to date a blind person. Prior to breaking up with him they were witnessed making out in the church's parking lot, while whispering "always" to each other. Which Augustus and Hazel took from them and created 'okay.' They said maybe 'okay ' will be our 'always'.

The relationship seemed very important to Isaac. She was only written about in a few scenes but was mentioned quite a few times by Isaac. Later in the book, Isaac takes revenge by egging her car with Augustus and Hazel. Isaac always talked about how he loved Monica, but then realized how she really felt about him, wising him up about the situation.