。
She's an adventurer to the bone.
Literal
She [topic-は] root-from [genitive-の] adventurer [is-だ].
根っから ('thoroughly, by nature, from one's roots') is built from 根 ('root') + っから (an emphatic from version of から) — literally 'from the root.' Modifies a noun via の: 根っからの+[noun] = 'a born X / X to the core / X through and through.' Common collocations include 根っからのファン ('die-hard fan'), 根っからの優しい人 ('genuinely kind person'). 冒険家 is the suffix ~家 ('-ist, expert, professional/dedicated practitioner') applied to 冒険 ('adventure') — same template as 小説家 (novelist), 政治家 (politician), 芸術家 (artist).