Final answer:
The figurative language used in the quote "But I am constant *as the Northern Star*..." is a simile.
Step-by-step explanation:
The figurative language used in the quote "But I am constant as the Northern Star..." is a simile. In a simile, one thing is compared to another using the words 'like' or 'as'. In this case, the speaker is comparing their constancy to the constancy of the Northern Star.