So my personal opinion. for leads, design all the way back to their childhoods, that's pretty essential. For main supporting casts I generally want to understand pretty far back too, things like parents it'll be worth noting the circumstances they came from that effect the way they raise their kids, for friends their backstories can effect why they are so attached to the leads, and the same with SOs. It's also just really helpful to have little details about the characters to fall back on to help the world feel fuller. It gives you interest to draw from which can be as simple as "if this character has a crush, are they more likely to compare them to an actor, a musician, or a cartoon character?" etc. details about their interests usually stem from things from their pasts, especially anytime anyone has a hyper-fixation with any one thing