"She is very much his type, physically. It's surprising that Kendall is being so public about this," the source says. "She's usually extremely private about who she's dating, but she seems really into Blake and she's actually happy, so it's making her relax a bit."
Jenner, 21, and Griffin, 28, have been seemingly inseparable lately, as they spent Labor Day weekend together at the Malibu Chili Cook-Off, and were photographed enjoying each other's company two nights in a row last week.
According to ET's source, however, while Jenner likes spending time with Griffin, she's not ready for anything serious, given that the NBA star has two children with his ex-girlfriend, Brynn Cameron.
"I highly doubt this is something that will last; it's just a fun fling," the source explains. "It's not like Kendall is going to step into a stepmother role. That's definitely not what she's looking for."