The best answers to this question are often those that are able to capture what makes you you. Most people who apply for TCs have decent grades, some work and volunteer experience, some internships and/or some university society experience. While these are things you do, they don't necessarily capture the essence of who you are.
You'll need to reflect on your personality and figure what those things are that you identify yourself with- it could be your creativity, resilience, ability to take initiative, ability to uplift your team members, enthusiasm, willingness to try new things etc.
Once you've figured out your USP, you now need to structure it within the context of the firm. Find those 2-3 qualities that most align with the firm's ethos and vision and then try your best to demonstrate this alignment in your answer.
A structure I'd recommend is to state you character trait/quality ➡️ talk about how you developed it and how you apply it in your life ➡️ link it with the firm's culture/vision/ethos/trainee characteristics to demonstrate your suitability to work at the firm.
