Take a look at our comprehensive guides to getting gold in, well, just about everything in GTA 5, here
This takes a fair bit of planning and patience; but can be done without too much additional effort. You’ll need our comprehensive Stock Market Guides to complete this.
From GTA Online or GTA 5 Story Mode, go to Pause => GTA Online => Swap Character. Then select the “empty” character slot. View the full guide, here…
Unfortunately, each character cannot have it’s own pool of funds. There is a workaround, but it’s not fun.