She is best friends with Margaret and works alongside her at the coffee shop that Mordecai and Rigby hang out at. She debuted in the episode Brain Eraser. She developed a crush on Rigby in the episode Do Me a Solid but at the time, he did not return her feelings nor did he consider her a friend.
In the episode Skunked, Rigby is sprayed by a wereskunk and would transform into a super-strong monstrous wereskunk whenever he was stressed or angered. He transformed into this beast and hurt both Mordecai and Skips and was sorry for doing so. When he poured coffee on himself in an attempt to remove the smell off him, he got hurt and Eileen couldn't help but laugh. He transformed again and almost hurt Eileen but after she tells Rigby he was scaring her and seeing everyone looking at him, he transformed back and apologized and was very upset for what he did and ran out, showing he did care about her.
Their relationship may have officially started in the episode, Camping Can Be Cool, when Eileen started a fire without matches and Rigby said that was cool. When Mordecai was trying to talk about constellations, Rigby and Eileen are playing video games and without even looking up, explains that the constellation was Ursa Major. Rigby then says "OHHHH! Man, she didn't even have to look up," making her blush. When Eileen breaks Margaret car window to escape the Deerman, Rigby again says she was cool breaking the window, though Eileen apologizes to Margaret.
In several episodes after that, it's stated that Rigby and Eileen double date with Mordecai and Margaret. The strongest support of their relationship is in the episode Diary. When Mordecai and Rigby accidentally destroy Margaret's diary, they ask Skips to fix it. Skips says that during the ritual, everyone's eyes have to be closed. Rigby opens his eyes briefly but this awakens the Guardian of Margaret's Secrets, a giant version of Margaret wearing a warrior outfit. Skips explains that to defeat the Guardian, it must must appeased by revealing personal secrets. Skips' secret is that long ago he had a lover who skipped with him everywhere they went. When she died, he vowed to allows skip to always remember her. Mordecai's secret was he took Rigby's extra bed for himself because it was so comfortable. Rigby, after saying many things, finally admits he thinks Eileen is hot without her glasses. The Guardian disappears and all its destruction is undone and the diary is fixed. The next day Eileen goes to work not wearing glasses, Making Rigby's jaw drop, though without good vision she bumps into the table.
In Death Bear, both Eileen and Rigby admit the other is cool.
In One Pull Up, Eileen tries to help train Rigby to do a pull up so he could keep his job but Rigby is still to wimpy to do it. Rigby then uses "The Russian" to use electro shock to make his muscles grow. His muscles then become to heavy and Eillen says there is no shortcuts to physical training but tries to motivate him to do a pull up. Rigy is finally able to do so but then has to be put into an ambulance. Eileen congratulates him and Rigby says he wanted to hug her but didn't want to crush her with his huge muscles. Eileen says she wants a hug and the two hug. Rigby accidentally crushes her but all she says was worth it.