All it would really have to do is show that Mordecai and Rigby are not the that Eileen points out that he and C.J. have not yet had a real date. Both of Rigby and Eileen's daughters are taller than both their parents. Bittersweet Ending: Mordecai and Rigby managed to find the letter and help Muscle Man get Official Couple: Rigby and Eileen are now officially dating.
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.
Regular Show S 06 E 28 Dumped At The Alter / Recap - TV Tropes
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.
And Benson has to stop Muscle Man from breaking that one too.
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Muscle Dad left Muscle Man a letter to open on his wedding day. By Mordecai when he learns about Rigby and Eileen.Real Date Part 1
Never Trust a Title: The title suggests that either Muscle Man or Starla would leave the wedding. Turns out it was Mordecai who disrupts the wedding and C. Rigby and Eileen are now officially dating. Benson's car getting wrecked again. This time by a turducken shooting out of a deep fryer and landing on it.
Not official yet but this episode definitely puts more strain on the Mordecai and C.
The episode ends with CJ and Mordecai deciding to temporarily end the relationship and the next episode has Rigby tell Mordecai to stop dating for a while so he can focus on himself, meaning that it will be a while before the whole "Mordecai must choose between CJ and Margaret" arc continues.
While Mordecai reads Muscle Dad's letter, most of it is Muscle Dad's voiceover, with a flashback of him writing it.
Muscle Man and Starla finally get married. Rigby reveals Eileen and he have been dating for awhile without Mordecai realizing it. J decide to break up for a bit because CJ still thinks Mordecai loves Margaret, but it's really because Mordecai is really bad at admitting his true feelings for C.