Jesse knows it's a stupid move to fall in love with Fiona. Again. He knows a relationship between them would crash and burn – perhaps figuratively, most likely literally. But despite what his rational side says, he's drawn to her smile and her vivacity, turned on by her competency and edginess. It reels him back in so slowly he doesn't realize it until he's wiping sweaty palms and licking dry lips, trying to tell himself to back off now.
By then, it's too late. He's hooked.
She doesn't notice it, thankfully. At least, she doesn't notice it this time. Jesse has had practice, and her eyes are firmly focused where Michael stands center stage.
Michael looks back with her same intensity, the kind that would burn Jesse alive if he ever came into direct contact with it. Michael and Fiona are a breed apart from Jesse. They're born to spy. They live and breathe secrets and betrayal and manipulation. Jesse may be good at it, may thrive on the action it causes, may have learned to admire the intricacies of deception – but at the end of the day he wants to pack it all away and be someone else: someone who can trust freely and act without secondary or tertiary agendas.
It's something Michael and Fiona don't understand. Jesse knows this, first hand. They didn't think anything of burning Jesse then earning his trust, not just as colleagues but as friends and confidants. The only reason they don't game each other is it would interfere with their main objective, i.e. their continued relationship, if they had different agendas.
It would be a rudimentary divide between Jesse and Fiona if they ever attempted a relationship. Jesse knows it would destroy them. He still can't help the way his heart rate picks up, or his mouth dries when she saunters up in one of her dresses, no matter how many times he tells himself not again. He knows how this will end, and he knows who Fiona will always choose. He has no chance at all but his traitorous mind still whispers what if.
Jesse tells himself to himself to fuck off, wets his mouth with a sip of water, and soldiers on.