it's an exaggeration he likes to run with for entertainment value but back in college stella had a crush on him and by the time he clued into that fact, they talked about it and decided there were too many complications etc. involved and they should stay friends. which they have. but she also ended up spending a year abroad after that for reasons not related, kel just likes to joke he's the reason she fled the whole ass country. honestly, they're very close friends and she might as well be family at this point.