"He grinned, and everything about him was perfect. And I fell in love instantly."
--Cecil, Episode 1: Pilot
Carlos Arrives in Night Vale Edit
Carlos arrives in Night Vale in the pilot episode, first introduced as an unnamed scientist. Cecil talks about his arrival with curiosity, wondering on air about his possible reasons for coming to Night Vale. It's also the first time Cecil makes mention of Carlos' "perfect hair" and good looks.
Carlos later calls a town meeting in the same episode, where Cecil is apparently present. It's there that Carlos reveals his intent to study Night Vale for scientific purposes, the town purportedly being the most scientifically interesting community in the U.S.. Cecil continues to wax lyrical over Carlos' beautiful appearance, particularly his wonderful hair (which becomes something of a running joke on the show) and, after Carlos flashes a grin at the assembled town's folk, Cecil declares that he "fell in love [with him] instantly!".
Cecil's Crush Edit
After the town meeting Carlos holds in Pilot, he is surprisingly seldom mentioned until the sixteenth episode, The Phone Call. In this episode, Cecil enthusiastically reveals that Carlos had called him the previous weekend. Though Carlos discusses time and the scientific abnormalities of Night Vale, Cecil focuses more on getting his crush's attention. While quoting their conversation, Cecil makes comments on Carlos's voice and intelligence and asks the listeners, "I feel like time always slows down when we're together, Carlos and I, is that what he's trying to say?" Cecil also asks the scientist if he "wanted to get together sometime and talk some more," which Carlos declines.
Later in the episode, Carlos calls back and Cecil plays his voice mails. Along with information about clocks, Carlos asks Cecil to meet him the following afternoon. Though the meeting is to discuss scientific business over coffee, Cecil calls it a date and comments, "it's just coffee, but maybe it's more! Maybe lots more."
Carlos Returns Cecil's Affections and Relationship Progresses Edit
After Carlos' near-death experience in One Year Later, he calls Cecil and invites him to the parking lot of the Arby's, where the two sit on Carlos' car, Cecil with his head on Carlos' shoulder and Carlos with his hand on Cecil's knee.
In First Date, Cecil and Carlos go on their first date. Cecil wears a tunic and furry pants. Due to the ban on wheat and wheat by-products, all that the restaurant can serve is mushrooms. Outside, the majority of Night Vale has become buzzing shadow entities. Once Cecil reaches Carlos' lab, Carlos says that he must do more tests to try and save the towns people from the shadow entities. Carlos rejects Cecil's offer to help, but then kisses him and leaves into his lab. Cecil goes back home happy and smiling.
Cecil and Carlos often referred to each other as boyfriends throughout the series and have been showing various moments of affection and worry towards each other's safeties. They also have kept a long-distance relationship when Carlos was stuck in the Desert Otherworld to the point of Cecil spending his vacation time in the Desert Otherworld. After Carlos' return to Night Vale, the two maintained a loving and healthy relationship.
Relationship notes on character pages were getting too long and involving too much episode content. As Cecil and Carlos' relationship is one of the show's main subplots, it should have its own section. Please post updates on their relationship here.
Carlos has many names for Cecil, including Cece, sweetie, my sweet Cecil, honey-voiced honey,  darling, caramel-voiced Cecil, Pooh etc.
It was revealed that Cecil is still harboring some trauma over the ownership of lot 37, and Carlos regularly offers words of comfort and empathy. Carlos reached him from the otherworld through phone, and it seemed Cecil was quite worried. Carlos also takes care of their food shopping, allowing Cecil to avoid the new auction system at the Ralph's.
While Carlos was in the Desert Otherworld, it is heavily implied that Cecil suffered from anxiety and depression, and his horoscope in Rumbling suggested that he try "taking up drinking while crying in a quiet room" (which means that he might have also had a drinking problem). He also repeatedly mentions that he felt like he was drifting apart from the other people of Night Vale. Also, in The September Monologues, Steve Carlsberg talks about how Cecil was much more focused on work in the past, and how "this past year has softened him a lot... In the right places", so it seems that, because of his relationship with Carlos, Cecil has become more outgoing and relaxed.
It has been decided that Cecil is, in fact, not moving to the Desert Otherworld where Carlos has recently been living. Instead, Carlos is moving back to Night Vale.
During a segment of "Hey There, Cecil", an anonymous questioner, signed "Lonely Boy In The Laboratory", asks Cecil out for an opera and drinks. Cecil responds that yes, he would like an opera, and, "whatever else after." 
In A Door Ajar Part 2, Cecil said that back when Carlos first arrived in Night Vale, they came across each other in the community college. Carlos took Cecil to the rooftop to show him a hazy column of darkness. Cecil pointed out his apartment building and Carlos complimented the roof. Cecil says it was the first time Carlos ever complimented him.