(warning: intense autism)
Marco and kelly have had an slow and mostly offscreen development, of which we only have glances of it in 3 episodes (a constrast of tomstar development)
Kelly seem to have developed a crush on him since LLB and MB while Marco still being an idiot about Star has show that he does like kelly, not in a huge way yet because he is a retard, (specially on MB were he dropped his annoyed face about Handsword dance to dance with kelly almost inmediatly)
I honestly think the next episodes will have more hints, until Star just realize their relationship have grown, like rigby and that mole relationship on regular show.
So maybe mid-late season 3b we could have a confirmation when star looks at them being cute and perfect togheter, which will only bring more drama and suffering, because Nefcy.
also sorry i wrote this on the shower.