In case anyone is curious, there is exactly one person in Biden's cabinet (16 people total) that previously served in Obama's cabinet: the Secretary of Agriculture. https://en.wikipedia.org/wiki/Tom_Vilsack
If you further consider "cabinet-level" positions (25 people total), two more people are included: the current Secretary of Veterans Affairs was previously the White House chief of staff and the current White House chief of staff was previously the acting director of the Office of Management and Budget. https://en.wikipedia.org/wiki/Denis_McDonough https://en.wikipedia.org/wiki/Jeff_Zients
Of the 5 people mentioned by name in the parent comment, 3 have never been in the Cabinet and none were in both Obama's and Biden's Cabinets.
My best guess for the unnamed person is the current director of the Central Intelligence Agency (previously a cabinet-level position!) and who was Acting Secretary of State under Obama? https://en.wikipedia.org/wiki/William_J._Burns_(diplomat)