Rex

In Xenoblade Chronicles 2, Rex was a somewhat scrawny 15 year old that got pulled into a conflict greater than himself. Flash forward to Future Redeemed and he's taken the lessons of his companions into adulthood and become a legendary warrior others can depend on in even the most dire of situations, while still keeping the heart of the salvager boy from Leftheria.

Example of:
Kid Hero All Grown-Up