I have a small biography within my main text that I use to "ground" my characters (and help me remember things). Based on what I have written so far, here is the bio of my first MC:
Cornelius Aaron Gault (1794-1883), aka “The Commodore”, was the founder of Gault Estate. He became socially prominent in the business world and amassed a huge fortune through steamboats, railroads, and various business enterprises from 1814-1824. He married Sophia Anne Gault. He discovered The Vault and The Machine in 1817 when building Gault Pavilion. His stories were first published in The Rise of a Railroad Tycoon and later compiled into The Gault Chronicles, both by James Anthony Wells.
Although this information is at present "just for me", I can use it to create an obituary when this MC dies (don't worry, there are other MCs).
I have compiled a simple genealogy in Excel which I can refer to and modify as needed. If I need ten extra years in a character's life, I can update all subsequent dates by 10 years to see how everything works. I have to be careful, though, because I am referring to actual historical events in the story and can't refer to something before it was invented or to a particular living person after they have died in "real life". For now, most of the genealogy is stable, but that may change as I work my way further/farther into the story.
I have blueprints of my MC's mansion and created a third floor (because I will need it for the plot). I have a map of the estate, as well, which can be changed on a whim. These will probably never be in the story, but I can see relations of rooms, buildings, secret tunnels, etc.
There is a lot of info on Google that you can use
for ideas, but which may required tweaking if you want to use them integrally in a story, such as pictures, blueprints, maps, writing samples and styles, etc. You can look into the Wikipedia biography of a person similar to your character and get ideas from that, changing and morphing it to fit how you perceive your character.
Of course, these are just my ideas. You may be creative enough to do all of this in your head, but this being my first story, I am just creating myself an environment where "my happy little characters can live" (Bob Ross).