Kellco is the romantic pairing between the characters Marco Diaz and Kelly.
In the episode Goblin Dogs while Star and Pony Head were catching up, Kelly and Marco spent some time together with Marco talking about his feelings. At the end of the episode, Marco tells her that "... it's been really great talking to you." Marco was startled when he met Tad and even asked him if he's been there the whole time.
In the episode Lava Lake Beach, both Marco and Kelly talked things out together after Marco saw Star kiss with Tom, and after Kelly breaks up with Tad. They decided to watch the Soulrise together and as they were walking, Kelly remarked “... Tad and I used to come here every Soulrise."
To this, Marco replied “... So, why come here now?”
Kelly answered “... I like the Soulrise, I don’t want it to be the place that makes me sad, so I have to face it. Re-write it so it was mine again.” Marco may not have visibly reacted but it's clear that it made some impact on him, as his overall countenance had improved since she told him; not only in the selfsame episode but in the episodes following. Right before the Soulrise, Marco gives his hoodie to Kelly, observing that she looked cold. However, Kelly then shared the hoodie with Marco. Following the Soulrise, Marco inquires about the time, and upon learning that it's after midnight, comments that it was now his birthday.
Kelly was the first one (and possibly the only one) to wish Marco happy birthday, as canon failed to show whether or not anyone else had expressed the same sentiment.
In the episode Monster Bash, Kelly asks Marco if he would like to dance with her. Marco accepts, and though they were only shown dancing together for a few seconds, it shows a quick change in Marco's attitude as he quickly accepts the dance and changed his facial expression for a happier one.
In Stump Day, Kelly attempts to calm Marco down and ask him to take back what he said to Tom about him being a bad boyfriend to Star. When Marco and Tom are fighting, Janna accuses Kelly of being jealous that Marco is fighting Tom over Star instead of her, which she denies but shows that she is concerned for Marco. Later in the episode, they are next to each other when they are warming themselves by the burning stump. At the end of the episode, the two are shown dancing together.
In "Booth Buddies", it seems as though Marco has taken Kelly there as his date. Marco also is shown taking Kelly to the wedding. Throughout the episode, she has various encounters with Marco's Dragoncycle. It is shown that he was to drive her home.
In "Divide" and "Conquer", both are part of his Marcnificent 7 in an effort to battle against Meteora.
In "The Ponyhead Show", Marco and Kelly are part of Pony Head's show as they do a cooking segment. Kelly gets upset by Marco's comment that wrestling is fake. Marco apologizes to her. After they don't end up doing their segment, Kelly invites Marco over to her house to cook.
In “Kelly’s World”, Marco and Kelly spend the day trying to run an errand, however, they keep getting distracted, as they learn to fight in sync, and Marco helps Kelly block Tad, who‘s been harassing her for weeks. Ultimately, after running the errand, Kelly proclaims them ‘Breakup Buddies’ and kisses Marco on the cheek, as well as holding his hand.
In "Cornball!", as Kelly is leaving to handle tensions between two teams in a game of 'Cornball,' she departs by kissing Marco on the cheek.
The canon probability for Kellco is relatively low due to the fact that it only recently got off the ground, and Marco and Kelly have broken up from being 'breakup buddies'. there is, however, a slight chance of them getting back together, as they remain friends.
The first factor is Marco's own feelings, as he had come to recognize that he has feelings for Star during Season 3, the earliest possible instance being that of the episode Scent of a Hoodie and the most obvious scenario being that of Lava Lake Beach when Tad gives him a reckoning. This obscures the relationship and or feelings Marco has with Kelly.
For instance, it is said that the 'Blood Moon's light would choose two lucky souls that danced beneath it would be bonded together for eternity'. For starters, after dancing beneath the light and despite being clearly chosen for being bonded, both Marco and Star did not stop to think of its implications; simply put, it was 'business as usual' for them. Second, while Marco had seen (or imagined or envisioned) seeing the Blood Moon after kissing Jackie (during the episode Bon Bon the Birthday Clown) it was his only encounter with it, besides the initial one at the ball itself. If it was only a 'reminder' that he should be with Star, it had failed to remind him of each and every case thereafter when he kissed Jackie, or even while dating her normally.
Third and most importantly, romanticism is a two-way street; it has not been shown to Star whenever she pursued someone romantically in the series. While Oskar is a somewhat minor character and things never became serious between them to warrant the Blood Moon’s envisioning, Star had re-entered a relationship with Tom during Season 3, obviously during the end of the episode Club Snubbed. She'd kissed him multiple times, yet, she'd never seen the Blood Moon appear to 'chastise' her as it had done for Marco during his sole instance. Furthermore, Star is rather wild and has an effervescent personality, and isn't much of a stickler for conformity. It's possible that even while she manifests feelings for Marco, she might not act on it if the plot-point of the Blood Moon was being the factor of decision.
Kellco has a chance of becoming canon, even if is a sideline pairing, as was Marco with Jackie or Star with Tom. As such this supports the theory that in the episode Stump Day, TomStar and Kellco are still developing. Although in honesty, there is the possibility that members of both pairs might begin to show jealousy of the other, per se, Star for example being jealous of Kelly's companionship with Marco. As shown in the latest episode, Kelly might have feelings for Marco but denies it. In other words Kelly is a tsundere type of girl. When Marco burned Tom, Kelly told Marco to take back what he said and when Kelly confronted Janna she told her that “Marco almost burned to a crisp”, showing that Kelly cares for Marco. After Booth Buddies, Star and Marco haven't addressed their true feelings for each other so Kelly inviting Marco over to her place was a sign of Kelly’s feelings.
In Kelly's World Marco and Kelly became breakup buddies since they started to developing feelings for each other. However in A Boy and His DC-700XE, they were revealed to have amicably stopped being breakup buddies.