Probably either Ysayle, Yotsuyu, or Venat in that order.

Jae obvi had legitimate feelings for Ysayle and her death felt undeserved and unnecessary in the grand scheme of things (I don't care what repercussions DSU implies) and would love a world where she can see the fruits of her own labor. I actually have an au where she lives and still play with the idea of making it canonical (despite the mess it would create with Jae and Esti's already messy polycule).

Jae doesn't have a particularly close relationship with Yotsuyu, but she always regrets the way her story ended. Both are victims of abuse,
but Jae turned out the way she did due to the love she revieved from others. Despite all of Yotsuyu's wrongdoings, Jae still wanted the opportunity to pay those kindness forward to someone who could've needed it. Her death felt all too short and undeserved, and it vexes Jae even to this day.

Venat's demise feels the most tragic in Jae'si's eyes. To my understanding, Venat was the only one of the Ancients not to rejoin the lifestream, effectively gone in every sense of the word. Had Jae'si the ability, she would at the very least try to bring her back into that cycle so that she could see the fruits of her labor. She deserves to see the result of her actions bearing not just strife and indifference, but community and love among the people of Etheirys. An edit to canon I might make in the future is keeping a shard of her essence in some form or fashion within the shards of Allegoria Jae'si uses for her weapons, as I feel Venat deserves at least some level of reverence as Azem does.