竟是

竟是 = adverb 竟 ('unexpectedly, against expectation') + copula 是 ('is/be'). It marks the surprise revelation of an identity: 'X turns out to be Y, contrary to what one would expect'. Equivalent to 居然是 (jūrán shì) but slightly more literary. Compare 竟然 (jìngrán) which works with any verb, while 竟是 is specifically the copula version.

mistakes

Don't confuse 竟是 with 就是 (jiùshì, 'precisely is') — they look similar but 竟是 carries a strong 'surprise!' flavor while 就是 is neutral identification.