I've tinkered with most story engines out there and, for me, the winner for crafting emotionally satisfying character arcs is a hybrid approach: use a strong planner like GPT-4 (via chat-based tools) to lay out the spine of the arc, then hand off scenes to something like Sudowrite or NovelAI for texture and voice.
When I say spine, I mean the classic beats — inciting incident, progressive complications, midpoint reversal, crisis, and catharsis — and how they map onto a character's inner life: flaw, desire, misbelief, choice, and consequence. GPT-4 is terrific at taking a high-level brief and turning it into a scene-by-scene outline that actually progresses a character, because you can iterate quickly: ask for a ten-scene arc, then ask it to rewrite scene five to escalate emotional stakes, or to flip the protagonist’s misbelief into an active choice. After that scaffold, NovelAI or Sudowrite shines by making the emotional texture sing; their tools are great for sensory detail, romantic tension, and creating recurring motifs that plant and pay off across a story.
A tip I swear by: keep a short character bible (three lines of core desire, core fear, key lie they tell themselves) and feed that with scene prompts. Use the AI to generate small micro-arcs inside scenes — a hesitation,
A Confession, a lie discovered — and then stitch those micro-arcs into the larger arc. For
romances, that means letting both halves grow: one may learn to trust, the other to stop running, and the AI can help you design scenes that test those lessons. Personally, this combo has helped me turn flat meet-cutes into full arcs that land emotionally, and I usually finish a draft feeling like the characters actually earned their ending.