Tessa is two years old when she takes an interest in Sebastian's guns.
He doesn't notice when it happens, just that it does. One day while he's cleaning his guns and has pieces occupying the space of the dining room table, there's bright blue eyes peering across from the other side. She doesn't touch anything, just watches, observing quietly.
The first time he notices her, he ignores her but watches her to be sure she doesn't touch anything.
The second time, she sits at his feet, blue eyes peering up through long messy blonde waves half tied back with a bow that probably cost more than most of his guns put together (He doesn't even want to know how much her little crimson dress cost).
The third time, he sits her on his lap and explains what he's doing every step. Tessa stays silent but grins widely up at him. He decides that he very much likes that smile, and presses a quick kiss to the top of her head.
By the time she is three years old, she can fully operate any of Sebastian's guns, and even has a collection of working guns scaled down for her use; Mummy's gift to her, which seems to make up for the fact that Mummy doesn't spend much time with her. He takes her for target practice, and soon learns that she's not a bad shot.
Sebastian realizes just how screwed up he is when he realizes that he's so fucking proud of her, rather than being horrified that his three year old daughter is better at shooting than many adults, but he honestly doesn't care.
She looks over at him and beams, her safety equipment just a little too big and looking lopsided on her head. "Did I do good, Daddy?"
"You did fantastic, sweetheart."
"Will Mummy be proud?"
Unlikely. Jim chooses when she wishes to lavish Tessa with affection, and when she doesn't want to even acknowledge the girl's existence. She yearns for Jim's approval so much; that he does find disturbing, and heartbreaking. "Of course."
Jim, for her part, watches the two carefully during the times she's at home and not busy with work. Sebastian sometimes catches her studying the little girl, her expression unreadable. Sometimes he thinks she may be jealous of how much like him Tessa is growing to be, not just in her hair and eyes – though she still has Jim's eye shape.
This is confirmed for him when Jim is particularly affectionate and handsy one night. Handsy naturally turns into much more, but it's different this time. Sebastian just knows it (not that he complains; Sebastian's not one to turn down a good shag). There's something in her eyes, a cold determination of sorts. He tries to push the thought from his mind and focus on the other sensations.
Still, he's not surprised when, a few weeks later when Jim is out undercover on a job, he wakes up to a plastic stick on the counter by the coffee pot bearing two pink lines.