so, i think my new headcanon for a mish-mash, catch-all version of ivy is this:
pamela isley was born to wealthy, distant parents in the 1930s. she lived a very sheltered, very controlled existence, groomed to be a Perfect Lady and Wife until she dared to be different and followed her own academic and independent dreams. she went to college in the 1950s and was largely shunned for being a woman in academia, but her mind was sharp enough to be begrudgingly recognized by her peers and superiors. she was socially awkward and almost always out of place, never wanting to follow the social norms and mores ascribed to women at the time, but unsure of how to be herself without being rejected.
this made her the perfect target for jason woodrue. she wanted his validation. he was brilliant and accomplished, and he paid her real attention. he acknowledged her mind and her brilliance. and by the time she earned her doctor of science in botany in the mid-60s, he had her eating out of the palm of his hand. she agreed to his experiments willingly the first time, letting him inject her with all manner of chemical and home-brewed toxin.
over the course of his experiments, she died twice. she spent four weeks in a coma and six months in the hospital. her parents never came to visit her. the experiments fundamentally changed her. but she was wild, raw, hurt and betrayed. she lashed out for years after leaving the hospital, blaming men for the manipulations of her mentor and humankind for its wreckage of the planet. when she confronted her parents about their distance, she ended up killing them.
the sexpot vamp routine is an act-- though second-nature at this point. she uses what men expect of her against them. it’s empowering and vengeful. the changes to her genetics have completely altered her into something not quite human and not quite plant. she ages slowly-- oh so very slowly. much like a tree. that’s why she still appears as old as she was when jason experimented on her decades ago, even though she is -- in reality -- almost 80 years old.
over the years her connection to the earth grew. her control strengthened. and her own genius in her field sees her continuing to experiment now, trying to find new ways to avenge the earth for its mistreatment by humans.