There aren't anyone with a decent political capital in his immediate circle (or anywhere else). It's kind of the point of being an autocracy — if such a person appears, it ends up humiliated/arrested or even dead.Anyone with a knowledge of Russian politics know of any actors who could be keen undermine Putin in this/have ambitions to succeed him?
Dmitry Medvedev was once considered to be a potential successor (and even got elected as a president when Putin needed a break demanded by the constitution), but after Putin decided to stay he saw his rising popularity to be an issue and made him a non-entity. At this point Medvedev is one of the loudest voices for the invasion, trying to earn back the favour of his moody master.
Sergey Shoygu is one of Putin closest allies, they love to spend their vacations together (their photos weirdly have that brokeback mountain vibes), but while he's generally respected for his service as a minister of emergency situations & now the minister of defense, he doesn't have any real capital as a politician.
The last big name that some considered to be a potential successor is Sergey Sobyanin, the current mayor of Moscow... but he doesn't have any ties to FSB or the army, and without their support he wouldn't be able to do anything even if he wanted to.