She astounded him. There wasn't a moment where Liam looked at Lydia and didn't wonder how her brain worked. She was always there for him when he needed her and Liam couldn't imagine a world without her.
Headcanons under the cut.
Liam clears half of one of his bookshelves for Lydia to keep all of her supernatural tomes and math textbooks so when she stays over she has what she might need
Lydia's presence calms Liam and she will place her hand on his chest to slow his breathing while she speaks about random things
They both cook a fair amount, though Lydia can pretty much only make three or so dishes from memory, the others she has to use a recipe book for and Liam has his grandmother's recipe book that he references often
They are both very close with each other's family, Lydia calls Liam's grandmother Mimi and Liam is close with Lydia's parents
Lydia really enjoys high up places when she's stressed and Liam, who is slightly afraid of heights, will stand in the middle of a rooftop while Lydia sits on the edge
Lydia will tell Liam she loves him all the time, not often in public as she's pretty uninterested in PDA but in private she'll whisper it into his skin and lips
Liam and Lydia will play Mario Kart and get really competitive, like screaming and shoving levels of competitive