Along with the former Winter Soldier, the new Black Widow and the deadly U.S. Agent, Fontaine has brought in “Black Widow” villain-turned-antihero Taskmaster, Yelena’s father Red Guardian, and the mysterious Ghost, who is now able to control her ability to phase through walls after the events of “Ant-Man and the Wasp.”

It's a valiant effort, but I think it's pretty clear who it is: Bob Reynolds a.k.a. Sentry (played by Lewis Pullman). Those who have been paying attention to Thunderbolts* will recall that Bob was prominently featured as a character in the first trailer for the blockbuster that arrived in September,