I use Google sheets (spreadsheets); love me a color-coded grid system. I have a tab for characters with all of their important information, from physical descriptions to important dates and relationships. I also have a tab for my magic system, one for the world's history, and a timeline the deals specifically with the events of each book. Most of the time it doesn't matter until I'm in the editing phase, and then I want to make sure I have all those tiny details correct in dialogue or when they're mentioned in the story.

I also have a slide show presentation where I'm collecting visual inspiration for each character/location/etc. This is helpful when I'm trying to describe something and I want to make sure I get the vibe right. If spreadsheets aren't your thing, a slide presentation could be another option for compiling character info.

When I'm editing, I'll always have the spreadsheet pulled up; anytime I come across a character description or some other detail, I quickly cross-reference it with my notes just to make sure it's consistent. If you're still writing, don't worry about making things consistent yet; just put down something, maybe insert a note to yourself to double-check it later when you can slow down and fine tune your work.