Alyssa, Jason and Seth became friends in the 1830's, after Alyssa met the Dawsons while travelling the country. Alyssa waited several years to disclose her connection to The Millers, by which time Jason and Seth have started considering her as a friend and ally, leaving them indiferent to this reveal.
Alyssa is first mentioned in Season One as Seth and Jason's friend, and it's revealed that Alyssa provided the Dawsons, mainly Jason and Seth, with the file on Andrei Dobremov. It is also shown that she has a rivalry with Alyson, who does not oppose the idea that Alyssa might be responsible for the body they found. This suspicion gets overthrown when Seth and Jason discover Andrei and Lincoln, a classmate, were working together to spy and eventually attack the Silverstones. Jason and Seth cleared Alyssa's name, and thanked her for her help with the file.
She is again brought up shortly after, having send over a birthday present for Jason, showing they know eachother very well when she gets him a rare edition of a book he likes. She apologizes for not being there.
Seth and Jason briefly consider tracking Alyssa and joining her when they are forced out of town by Payton, but trouble with Lauren and Nate forces them to stay, and eventually they choose to continue attending SJBS.
Alyssa informs Seth and Jason of the impending attack of The Travellers, giving the alliance between the Dawsons and the Silverstones an advantage. However the fight is still a equal match, and Alyssa eventually arrives just in time to provide assistance. When three members of the travellers are able to escape, she ensures she will go after them. Jason and Seth both ask her to stay around, but she chooses not to because of Alyson and Dylan. Later, Seth explains his relationship with Alyssa to Claire.
The three of them stay in touch while Alyssa travels the country, hunting down the remaining travellers despite orders from her mother not to. She tracks them down in Paris, which is where Payton, Jason and Seth travel to when Alyssa gets called in by her mother for breaking the rules. However Paris turns out to be a trap, set up by Aria Woods to capture Jason for breaking the rules. Seth goes after the anconi trackers while Jason (along with Payton) fight Aria.
When they're finished in Paris, Alyssa talks to Jason about bringing Payton. Alyssa's conflicted feelings about the alliance with the Silverstones peak through. Alyssa feels betrayed because of Seth and Jason's plans to ally with the witches knowing that Alyssa could never be a part of that alliance due to her loyalty to the Millers.
These feelings return when she arrives late June, joining in on prom with Seth, Jason but also Claire, Payton and their human friends. Alyssa is anything but pleased about not being able to have fun (as a vampire) and dismisses the friendships that Seth and Jason have made at the school. Jason then shows Alyssa the carelessness of the humans that he envies, and Alyssa agrees to a dance with Justin. Shortly after she leaves the party however, only to find one of the humans fatally attacked. Knowing who was behind the attack, Alyssa saves the life of Katie Sampter and takes her to safety. This means she has to lie to Seth and Jason when asked if she knows anything about the attack, and they believe her.
However when Claire gets missing, Alyssa recognizes the concern Seth shows for her. Alyssa then reveals to Jason she knows who is behind the attack. Explaining things to the Dawsons, Jason and Alyssa take charge in tracking down Jared but he escapes because he knows their weaknesses. Meanwhile Seth feels left out by the secret that Jason and Alyssa kept, and focusses all his energy on saving Claire instead.
Alyssa seemingly comes around on the way she thinks about the alliance with the witches when Seth is watching over Claire, and she asks Jason to update her on Claire's fate.
Jason and Seth communicate with Alyssa about Claire's recovery and Jason's plans to move to Portland to help Payton out. Meanwhile Alyssa hangs around at the Miller compound in an effort to confrot Jared for what he did to Claire, telling Seth that if Claire is important to him, she is important to her.
Alyssa then gets word of an upcoming attack through Matt, but Alyssa is too late to stop the attack and she arrives right after her cousin Liam Miller has been sent to Waterfall Creek. When Seth is willing to kill him for trying to hurt Claire, again, Alyssa convinces him to trust her stating that Liam was just following orders. When Jason asks if there's any more family members he should know of, he and Alyssa laugh about it together.
Alyssa and Seth discuss Claire being the target of another attack, and Alyssa states that her mother has crossed a line using Liam's biology like that. She states that she will head back to the Millers to talk to her mother. Seth sees the dangers in it, and Alyssa thanks him for being worried. She then goes to see Jason, and when she finds out he is packing she advises him to stay, knowing that James will need strategic intel to help protect the town from The Millers. Alyssa hints that she knows Jason wants to go to Portland to be with Payton for alterior motives, but doesn't seem to object to the idea.
Once back in the Millers, Alyssa disrupts an important meeting of her mother and attacks several highly ranked vampires to cause chaos in the vampire political world. In doing so, Alyssa gets taken down and she is poisoned before being let go. She arranges a meeting with Jason and Seth in Portland, claiming to have intel from The Millers. Instead, she shoots Jason and Seth with poison arrows and goes after Payton who is protected by Lauren and Nate, who take her out.
Alyssa is cured with the help of Claire, who states that Seth refused to believe Alyssa had gone bad despite shooting them. Alyssa thanks Seth for getting his girlfriend to cure her, to which Seth doesn't deny he has feelings for her.