After his talk with Tad, Marco realises his feelings for Star. When he came out from Kelly's hair, he was in a emotional dilema, watching people together, kissing, holding hands, until he arrives on a side of the beach. Then suddenly he stops and in front of him, he sees that Star was kissing Tom and is when he realises that he was too late. So after that, he went to sit on a far away on the sand, and then Kelly appears behind him and telling him that thanks to him Tad finally moved out from her hair. Moments later, he went to see the Soulrise with Kelly and she wishes Happy birthday to him.
On Mewni Castle
After Marco saw the Soulrise, he returned at the castle alone, because Star was with Tom. And she didn't bother to say Happy Birthday to him. He went to his room and open the door and immediately closed it, sad and heartbroken went to his bed.
Marco's thoughts: How I couldn't realise my feelings so late? Now I lost my chance to be with Star. I'm so stupid. Now I know how Star felt when I was with Jackie, but no matter what she was still by my side supporting me even though she was sad and heartbroken inside even she faked smiles when I was with Jackie, and it was all my fault for not realising how she felt, but when she confessed that she had a crush on me, I stayed without saying a word and meanwhile she left crying because she left because of Toffee returned and I didn't give her an immediate response. Of course I liked her in the beginning but after the Blood Moon Ball, when she told me that she didn't need a hero, she needed a friend, that sentence broke my heart and since that day I focused my crush on Jackie and forget my feelings for Star. I guess it will be better if I hid my feelings for her and even if breaks my heart seeing her with Tom, I will protect her as her squire and continuing to offer my friendship to her.
After he thought about that, he felt sleep, crying and with a broken heart.