i would definitely say Lizzie as well. BUT, I think Vanessa might be the mom instead. Similar looks, tattoos one arms. Timeline would fit. She said she would be turning 21 soon after all
This is my convoluted head-canon, but there's a reason Skylar went out of town randomly and came back with sleeves that look just like Lizzie's, acting cryptically toward the MC. Which led me to believe that that Lizzie was sent there rather than being from a "different school" to watch the MC, and that she knows something about the power plant "machine".
** RECENT UPDATE SPOILERS**
This was confirmed to me by the last two people you see in the final scene of the most recent. Sky recognizes you (which tells me she got involved w/time travel since it's 20 yrs in the future and she's not old), and Lizzie doesn't (which tells me this is a different version of the girl who sat next to you in the one class and seemed to admire you). So I think she's a plant, put there by one of the powers that be--OR she's actually a 19 yr old Riley sent to watch him, bc why not? It's time travel in an AVN lol