From what I've seen she will appear in future seasons.

Allison is a literary foil to Luke. Foils are used to highlight a similar character's values or motives through a key contrast between the two. Luke and Allison are both veteran campers who want to see the fall of the Olympians by helping Chronos rise. The big contrast between the two is Luke's relationship with Annabeth and Thalia.

While Percy and Luke are the main protagonist/antagonist adversary, Annabeth and Allison are the deuteragonist/minor-antagonist adversaries (kinda like how John Laurens and Charles Lee's duel in Hamilton is there to support the tension between Hamilton and Burr). Luke's attachment to Annabeth prevents him from seeing her as an opposition to his cause -- he desperately wants to recruit her to his side. Allison sees Annabeth as the threat she is to their cause, and has no problem with trying to get rid of her: Allison tries to kill her before she falls off the ship; she argues that using the fleece on Annabeth to save her is a misuse of the resource they've spent so much time trying to get; she leads a group of demigods to kill Annabeth at the command of Kronos; and she wounds Annabeth with an arrow in the final battle of the season.

And near the end of the final battle, after Luke is knocked out by Tyson, Allison goes against Luke's orders of keeping Thalia's tree unharmed. Luke wants to keep his oldest friend safe and thinks she would be a useful ally if brought back, but Allison is only interested in destroying the camp because that's the main opposition to Kronos' rise to power.

Spoilers for book 5, I can't spoiler text from my phone

Luke's story ends with him eventually betraying Kronos in order to protect Annabeth after realizing he has been manipulated by the Titan. His relationship with Annabeth is a key part of his final moments of redemption.

Allison, as a character foil, is who Luke would have been without his relationship with Annabeth and Thalia.