Kruge is a Klingon Defense Force officer who tried to take the secrets of Project Genesis for the Empire. During a fight with Admiral James T. Kirk on the rapidly-disintegrating Genesis Planet, Kruge was knocked over the side of a cliff and appeared to fall into a pool of molten lava. (Star Trek III: The Search for Spock)
However, before he hits the pool, he crosses through the intervening 96 years of time via a spatial anomaly formed by Genesis' collapse (the so-called "Fountain of Possibilities") and reappears in the Main Engineering compartment of the USS Jacqueline Dorsey.(Harry Potter and the Fountain of Possibilities: "Investigation")
Arrangements between the Federation and the Klingon Empire are soon made for Kruge to be transported to Qo'noS by the IKS Gorkon, accompanied by the Jacqueline Dorsey and the USS Enterprise-A. (Harry Potter and the Fountain of Possibilities: "The Fire Inside")
After reading an information packet on the major events in galactic history following the Genesis Incident, Kruge expressed to Captain Kirk that he regretted his actions 96 years before and looked forward to working to earn Kirk's trust. (Harry Potter and the Fountain of Possibilities: "All Hell Breaks Loose")
As promised, Kruge would proceed to do just that: at a dinner party held at Klingon Chancellor Martok's estate, Kruge overheard Ron Weasley and sister Ginny Potter discussing their worries that Ambassador Harry Potter and head of security Lieutenant Commander Hermione Granger would still fall in love.
Kruge informed the Federation Embassy on Qo'noS of what he had discovered, leading Harry and Hermione to meet up with Kruge to learn about the situation in person. Subsequently, Kruge joined Harry in supporting Hermione's blood oath to find the culprits responsible. (Harry Potter and the Fountain of Possibilities: "Wrong Place, Wrong Time," "Right from Wrong")
To that end, Kruge joined Harry and Hermione in traveling back in time to Harry and Hermione's second year at Hogwarts School of Witchcraft and Wizardry. (Harry Potter and the Fountain of Possibilities: "Honor's Blood," "The Truth Comes Out")