The best tip I can give is to build your original character from the ground up, but not by filling out a massive template. Start with a core contradiction. Something like 'a knight who's terrified of horses' or 'a healer who is secretly poisoning the town's water.' That internal friction gives you an immediate motor for scenes. Then, let everything else—their voice, their past, their habits—sprout from that seed.
I find it helps to write a few mundane scenes that never make it into the roleplay. What does your character do on a lazy afternoon? How do they react when they stub their toe? Those tiny, quiet moments reveal more than any grand backstory. It forces you to think about their instincts, not just their plot function.
Also, don't be afraid to let them be inconsistent. Real people are. Maybe they're bold one day and a coward the next, depending on what's at stake. Your writing partner will likely find those wobbles more relatable than a perfectly predictable archetype. The goal isn't to create someone 'likable,' but someone who feels like they could walk off the page, even if they'd be a jerk.