Markapoo is the ship name for Marco and Hekapoo. While there is an attraction between them, it's when Marco's adult self (or voice) is involved.
Its probability is only at around less than 0% due to the lack of on-screen moments. Plus as the series has one episode left before it ends, and Starco is already in effect, a quick last minute relationship would be a dumb and stupid idea.
For Marco, Hekapoo is the person he's spent the longest amount of time in comparison to Star, Jackie, and Tom. 16 years, in fact, which implies they've gotten pretty close over that amount of time despite it only being 8 minutes of Earth time.
During "Night Life", Hekapoo gets mad that Marco doesn't reveal that the rogue portals are being made by Star. She implies that they are done working together. This appears to have damaged their relationship further.
And also Hekapoo has been erased from existence in "Cleaved", so canon probability drops to zero. Even if she were alive, it's too late, since Star and Marco are dating.
List of Markapoo moments
In Running with Scissors, an older Marco seems to be flirting with Hekapoo as if they've gotten to know each other over the past 16 years in that dimension, or 8 minutes in the Earth dimension. This implies they may have been forming a bond over the majority of his life (and a nominal but memorable chunk of Hekapoo's ancient existence).
During the second live chat, Hekapoo appears three times. The first two times she smacks his head playfully. After the second time, Marco says that he might go visit her soon. At the end of the live chat with Marco, when he wishes Happy Valentine's Day to Star and Jackie, he is taken by Hekapoo into her dimension (presumably so Hekapoo can enjoy Valentine's Day with adult Marco). Theories show that Marco left for three to five seconds, equivalent to two to three months, meaning she had two to three months with Marco.
In "Night Life", Marco helps Hekapoo deal with rogue inter-dimensional portals. Marco enjoys this time spent with Hekapoo so much that it is in equal measure the reason for his silence, alongside protecting Star from magical commission inquisition. Their rendezvous point is a roadhouse or tavern where intoxicating drinks mentioned as partook of after their escapades. Two adults, with needs and well past the ages of precocious sexual modesty, with very strong mutual attraction, a working partnership with intuitively coordinated interplay during strenuous activity, alcoholic/(inter-dimensional equivalent) libations toasting each night's completion, in an anonymous roadhouse with inn style room accommodations--it is not a question of IF they have slept together, it is a question of HOW MANY times; and perhaps whether they agreed to wipe his memories--So that his older self's romantic experiences didn't complicate his younger self's life. When she finds out that Marco knew the stray portals that they were battling were being created by Star in her sleep, Hekapoo becomes angry with Marco for not telling her. While she agrees not to tell the rest of the Magic High Commission (after claiming she couldn't resist Marco's adult voice), she says that they are done working together. This may have damaged their previous relationship.
Popularity in the Fandom
As soon as this ship was introduced, there were many people who instantly latched onto it. There are also many who partially support this or at least casually enjoy it. This may be because of Hekapoo's natural, playful, fierce-yet-passive appearance and personality.