Describe...
1: their voice 2: their smile 3: their greatest achievement 4: their insecurities 5: their shortcomings 6: how they deal with grief 7: how they like to dress 8: what they like to eat 9: their theme 10: their fashion sense 11: their family life 12: their romantic life 13: their embarrassing memory from years ago 14: how they react to burning their tongue on food 15: how they react to a brainfreeze 16: their dreams 17: their ambitions 18: how they sleep 19: their reaction to betrayal 20: their reaction to a mystery love letter 21: how they react to pain 22: what they're like on two hours of sleep 23: how they act when they're sick 24: what motivates them 25: why you enjoy them
Try write about a paragraph on each item to further understand your character.