Usage patterns
意志が強いto have a strong will
He has a strong will and sees his goals through to the end.
意志が弱いto have a weak will
The reason I can't keep up my diet is that I have a weak will.
〈人〉の意志でof one's own free will; by one's own decision
She quit the company of her own free will.
意志を持つto have (a) will/resolve
I took on the new job with a strong sense of determination.
Expressions
意志の強いstrong-willed; strong-minded; having great willpower