Horrible fact about Fool’s Gold: Rem still gets a crush on Subaru.
Long story short, Heinkelbaru manages to help Rem like Subaru does in canon around the time Arc 3 happens. But while Heinkelbaru is incapable of seeing Rem as anything but a child he’s grown fond of (because 1) Rem is a year younger than his son, Reinhard, 2) Heinkel is married and 3) Subaru is still wildly underestimating people’s ages and thus thinks that Rem is somewhere around 9 years old), Rem has absolutely no qualms about falling head over heels for that 40-year-old man.
Heinkelbaru doesn’t even notice. He still gives her headpats when they meet and largely treats her like canon!Subaru treats Petra. Everyone else very much notices, because Rem is not even trying to be subtle. Not like she’d be very good at it anyway but she’s Not.
Also Heinkelbaru is still kind of flirting with Roswaal (Rem’s boss) just to blow off steam and Rem is totally responding to that like a normal person. (“Just like Nee-sama, I see! Heinkel-san truly has superb taste!”)