Describe someone who stays "True to themselves"
Solution 1:
You might say the individual didn't compromise his principles or didn't give in to pressure.
Alternatively, you could say he persisted or persevered (in his beliefs).
Potential adjectives for such a person are incorruptible, upright and steadfast. (Obviously, there are many others).
A casual phrase that expresses this idea is to stick to one's guns.
Solution 2:
- principled
- undeterred
- indomitable
- resolute
- staunch
- true blue
- Spartan
- undaunted