Gautam had a son called Sharadvan – who was supposedly born with arrows. He was great archer and attained the art of all types of warfare. He bore twins – a boy and a girl – from an Apsara called Janapadi. The twins were found by King Shantanu, who brought them to Hastinapura and called them Kripa and Kripi. Kripa went on to become the high priest of Hastinapura, and also the early teacher of the 100 Kauravas and 5 Pandavas.
Kripi married Drona, and gave birth to Ashwatthama. So Kripa was also maternal uncle of Ashwatthama and guru.
Kripacharya was an extraordinary teacher. To him, all pupils were equal. A guru like Drona favoured Arjuna and sacrificed an equally if not more promising student - he asked Ekalavya for his right thumb. Kripa, on the other hand, upheld the highest standards expected of a teacher: He was completely impartial.
He fought on the side of Duryodhana in the War, also was part of the murder of Abhimanyu and the manslaughter at night that was orchestrated by Ashwatthama. However in all these cases, his part is always that of a bystander and the one who goes with the flow.
Very little is known about why he became a Chiranjeevi and what is his status in the later ages. I intend to find more and post about this soon.