Hm...maybe...Gardevoir?

My loyalty is unquestionable
Multiple pokedex entries say that it will give its life for its trainer. That's about as loyal as it gets.

My look is undeniable
Um...everyone knows what a Gardevoir looks like?

My power is incredible
Again, does this really need explanation? It's one of the stronger Psychic-Types.

On all accounts, I'm liable.
Perhaps this refers to its ability to predict the future and responsibilities that wielding such a power holds.

For my trainer, for my heart...
For the light that I defend
For all that I hold dear to me
The violent will meet their end!

Again, the whole giving-life-to-defend-those-its-holds-dearest thing. Also, it's younger forms are said to gravitate towards trainers with positive emotions; that might be 'the light that [it] defend.'