Steve and Kristen were passengers killed in the crash of Flight 815. They are mentioned in "Walkabout." Steve and Kristen were engaged and were prepared to marry. They were sitting a "couple of rows" behind Claire.
Blooper
In her eulogy for the pair Claire says "I don't know their last name, but they were really in love and were going to be married". As they were not yet married she should have said "I don’t know their last names".