Ryan started out a temp at Dunder Mifflin, while going to business school. On the first
day of work, Michael tried to get 
him to pretend fire Pam as a joke; she ended up bawling,
and Ryan realized what he had gotten himself into. Michael has a huge non-sexual (I think)
crush on Ryan, and has made it know in many ways (calling him incessantly and pretending to be
Mike Tyson and giving him the "Sexiest in the Office" Dundie Award). 
Ryan started
dating Kelly after he found out that she liked him; of course, Ryan just wanted to sleep around,
while Kelly wanted to marry, and have lots of babies.
Kelly Kapoor is the annoyingly loud, shallow, and fashion obsessed diva of the office. She works in Customer Service, even training the accountants in that department for the time that the company faced a crisis (over boxes of papers being shipped out with a watermark of two cartoon characters having sex). Kelly doesn't believe in being honest with her feelings for other people, but would rather play games with her men.
Ryan was dying for a way out out the relationship with Kelly, and got it when he
was offered a job at Corporate with Dunder Mifflin. As soon as he hung up the phone,
he said to Kelly, "You and I are done!" Of course, 
Kelly didn't give up so easily,
telling Ryan the next time that she saw him that she was pregnant. Of course, she wasn't, and
that alienated Ryan even more.
Ryan eventually started a new website at Dunder Mifflin, which was his brainchild, but it ended up doing poorly (Dwight, thinking that the website had become self-aware, beat it in a contest on opening day). Ryan started to fudge the numbers, and was eventually arrested for defrauding the shareholders. However, he was somehow paroled and returned to work at Dunder Mifflin as a temp.
However,
Kelly at the time was with Daryl from the warehouse. Ryan was able to do a number of push ups in front of Kelly's
cubicle, which led to them getting back together. Kelly then texted Daryl that she was breaking up
with him (at Ryan's insistence), and 
his non-chalant response ("That's cool") made Ryan realize that
once again, he had made a huge mistake and was stuck with Kelly. Of course, he realized,
"I just can't do better than Kelly". At the last episode, he told
her that he was leaving alone for Thailand, and wanted all of her money, as this was
a once in a lifetime opportunity... Kelly reluctantly agreed...