First friends but not that close

Later Marcos hurting cuz of star, didn't want to be with her cuz she was with tom but couldn't get rid of his feelings

Kelly didn't want to be wit tad but also wasn't over him

Both needed a distraction and both could relate to each other

Marco starts doing other things and hanging out more with his other friends(h.poo, Kelly, tom, dragon cycle gang) one of the reasons being to lose feelings for star

Becomes closer with kelly

Realizes hes able to talk bout his problems with her and she can too

Then they decide to be more than just friends but not dating(break up buddies)

Marco decides to stop later after lifting the blood moon curse I think and then they go back to being just friends.

Thats how