The Martin family was a family of witches, and one of the main families in Season Two.
Jonas Martin - was a medical doctor that moved from Louisiana with his son, Luka. He collected Grimoires of witches which he says are all his families', saying "witches are family. We’re all bounded together by code of loyalty to help each other". He was working with Elijah in hopes of being able to free his daughter Greta Martin from Klaus. Jonas has a deep love for his children, which is seen, along with some possible anger issues, when he goes on a destructive tirade at The Grill after Luka is killed. He attempts to kidnap Elena to bargain his daughter's release from Klaus. Jonas was killed by Stefan Salvatore and Katherine Pierce.
Greta Martin - was the daughter of Jonas and sister of Luka Martin. As a witch, she was allegedly kidnapped by Klaus, as were many others, and forced into finding another way to break the Curse of the Sun and Moon without using a Petrova Doppelgänger. However, it seems as though her allegiance to Klaus is more intentional than obligated. Greta was later killed by Damon Salvatore during the sacrifice.