He doesn't know why he started babysitting - and certainly, if Liz knew the kind of life he lived within the Brotherhood, perhaps she would've thought twice about him taking care of her 5 year old son. But he and Liz had a sort of odd relationship. One based on Lara, which made it more complicated. Connected by grief, by a deceased wife and a close friend. And now it's just them. And Eddie, of course. Eddie keeps Mars on his game, keeps him focusing on the little boy who's precocious and playing with LEGOs. And Mars focuses on that, focuses on showing him that if he really wants the building to be structurally sound, you'd put the blocks here, not there. It's easier to keep busy when Eddie's running around and asking questions and doing the most. He likes this kind of chaos.
And by the time Eddie's asleep in one of the guest rooms in Mars's architecturally impressive home, he's moved back into the cozy den. Music is playing quietly and he's ordering Indian food for them to share, knowing Liz will be back soon. He walks himself over to the door when he gets a text, not wanting to ring the doorbell and wake the sleeping child. "Hello babes." He murmurs, leaning to give her a kiss on the cheek before letting her inside. "Eddie's already asleep. The high-rise he built is pretty impressive though. On the coffee table." He points with a grin.