What you need to know and what you want to know are two separate things.
He was the same as every other popular boy at school, leaning towards the dark side, slept with lots of people, everyone's date to the Yule Ball until he stood them all up and didn't go at all. This habit didn't wean in adulthood, when you were a metamorphmagus you could do so many things. Lukas, who thought the world owed him a living, was simply never going to settle down - so perhaps he should have been a little more careful before passing down his narcissism and his abilities to a child. He'd changed his name and ditched his old family, his brother, all of them.
Waltzing into an Oslo nightclub in another one of his disguises, he sought to satiate his desires, or many of them. Lukas
Rybak Sørensen was an arrogant man, who enjoyed manipulating people to fulfil what he needed in the moment - but even for a man like Lukas some things occasionally didn’t go to plan. What exactly didn’t go to plan for the trickster? Running into Marina Himmelreich - he didn’t know she had one child, six year old Alexandrine, being looked after by a cousin at home in Munich. He didn’t know that he’d have to take the child, because she’d be the first to vanish without a trace.
He was not father material, he knew that without doubt, but if she wouldn’t take the girl he could “adopt” her into his “family” and raise her in the ways he had been taught - at least until she was more trouble than she was worth. Unfortunately this never seemed to come to fruition, and all she really did was idolise him out of all of his goals, and she went from person to person in his circle whenever he did something out of place.
His lust was proved by that one night with the German model, Marina. He never did find her, not that he cared, he just wanted to remove that one mark on his record - mostly because she died quite young. Who he did find was the girl’s young daughter prior to Marina’s death, but she seemed all too indifferent and already experienced in the subject, nor did she seem ready to reveal much. However, somehow he did manage to leave Astrid behind in England with her - by some miracle he'd managed to coerce her into taking Astrid, maybe because Alexandrine herself had had quite the deadbeat dad.
Arriving in England was not as pleasurable as Astrid had hoped, although her sister was tolerable (and quite pretty). Hogwarts is something she's yet to get used to after so long in Norway and in Durmstrang.