Start with a few words to describe your OC, and then concentrate it downwards, like a funnel.

We'll use Gale as an example. Gale is energetic, friendly, and helpful, what you'd look for in a mascot.

Then, from each word, delve a bit deeper and narrow it down. Well, Gale's energy helps counteract Sage's tiredness, but they can also be quite annoying and hyper.

Add details to what you've got from there - Gale, already being naturally hyped up, can't handle anything like caffeine or chocolate.

And already, we've made great strides to making an OC more believable and deep