Steve Van Gerbig is the twin brother of Kwan and son of Lane and Zack. Lane thought Mrs. Kim would be mad when she told her she was pregnant with twins, but Mrs. Kim viewed it as a blessing. Kwan and Steve were born in late season 7. We see them again in Season 8.
[Rory, Logan and Zack talking in Luke's Diner]
Rory: "Hey, we just saw Kwan and Steve."
Zack: "Oh yeah? Wait, Lane was there too, right?"
Logan: "She was there. Looks great."
Rory: "And the babies are already growing. They're so big."
Zack: "I know. huh? Especially Kwan. Well, at least in the torso. He's got this, like, super long torso but Steve's really growing in the arms and legs department."
Rory: "I noticed he has long legs."
Zack: "Dude, Steve's an octopus, man, but Kwan's got the torso. When it comes to the torso, he's so on it."