Leonard met Stephanie Barnett in "The Lizard-Spock Expansion" through Howard, who had earlier met her in a club. Howard told her she can drive a car on Mars {the Mars Rover} if she would join him. Eventually, that plan backfired. After Stephanie's last appearance in "The Vartabedian Conundrum", it seems they have broken up when he asked her to move out without any further explanation. Good chemistry, but too fast.