。
She is more important to me than my own life.
Literal
She [topic-は] my life [than-より] important is.
A heightened, formal statement of devotion. ~である is the literary/formal form of the copula だ/です — markedly elevated, common in essays, declarations, and dramatic prose, but unusual in casual conversation. 命より大事 ('more important than [my] life') is a stock Japanese idiom for ultimate value, often in romantic, parental, or vow contexts. The combination of formal ~である with this lofty content gives the sentence a vow-like or declarative weight.