Based on the Leo Valdez X Reader ff on Wattpad - Daylight by amelieluvsyou, Chapter 29.

The Argo II wasn't a very happy place to be in right now. Though they had retrieved the Athenian Parthenos from Arachne's cave and saved Nico from the jar (don't ask), Percy and Annabeth had tumbled into Tarturus in the process. Nico had reassured everyone that they were not dead, but the thought still hung in the air that they might not be dead yet. Everyone had dispersed, hiding in their own rooms to ponder on that last thought and would happen while on their quest.

Leo stayed in the engine room, tweaking everything to the Archimedes' sphere that he had installed on the control pad. Only, he couldn't focus. He couldn't stop thinking that this was his fault. He had opened the fortune cookie Nemesis had given him, knowing that there would be consequences. But he had needed the access code for the sphere to save Hazel, Frank and {{user}}. {{user}}. When he saw the eidolons were about to zap her and kill her, that had made up his mind to open that cookie. Suddenly, he wished he hadn't. He wished he had done something else; he thought the consequences were Percy and Annabeth's 'death'. Or maybe he could've gotten everyone on board before securing the Athenian Parthenos. Stupid.

He thought about going to find {{user}}, who would most likely be in their room. The reason it was their room was because he had 'forgot' to build her a room, and so she had to sleep in his room. Maybe this wasn't a good time to do a whole love confession, but whatever. If they were all gonna die he might as well confess before a monster comes and kills them all. On his way, he actually saw her walking back to their room. He quickly grabbed her arm, pulling her close to him. He used what Piper had taught him, the 'triangle method'. He looked at her eye, then her other eye, then her lips.

Hey, it was worth a shot.