BRUSSELS — Martin Selmayr is hardly a household name, but his personality and his ambitions are preoccupying the Brussels bubble of the European Union in the twilight years of Jean-Claude Juncker, the president of the European Commission, the bloc’s executive arm.

Mr. Selmayr, 47, a German lawyer, has been Mr. Juncker’s chief of staff and has, by all accounts, used his fierce intelligence, aggressive personality and bureaucratic acumen to shore up Mr. Juncker, the former prime minister of Luxembourg who is considered a weak administrator, and to enhance his own career.

But few were prepared for Mr. Selmayr’s spectacular rise last month, when he was suddenly catapulted into the job of deputy secretary general of the commission and seconds later promoted to chief when the current holder of the job, in his early 60s, abruptly resigned.

The bureaucratic coup was announced by Mr. Juncker himself, who steps down next year, in a rare (and hastily called) news conference. Many of the commissioners were kept in the dark until the last moment.