Jink is a student in Astrid's AP math class. It is unknown whether Jink is a male or female. Jink is fifteen so he/she poofs out with the other adults when the FAYZ is created.
Astrid tells Sam and Quinn that Jink, and the other student Michael, have dyslexia. This is why they are in a class with Astrid, who is younger than they are. Astrid claims that they are both "math whizzes" and are better than her, which is significant for Astrid to say.
It is the disappearance of Jink and Michael, along with the adults, that first leads Astrid to make the connection between the poof and the age of those fifteen and over. Jink is not mentioned for the rest of the series but it is presumed that he/she continues life as normal on the outside.