The Gentleman by Theresa Beachman

The guy who’ll die for her.
The brother who’ll kill for her.
The woman who can damn well handle herself.

Leonid Bychkov has always lived in the shadows—first as part of a secretive special operations unit feared for its lethal precision, now as the formidable head of Guardsmen Security. Haunted by a past steeped in violence and missions that never officially existed, Leo doesn’t believe he deserves redemption—or love.
Especially not with Katarina Landon, the fiery MI6 officer who’s always been just out of reach—and who sets his blood ablaze like no other. When he receives an interrupted message from her, he drops everything. He’s heading to England, and he’s not stopping until he finds her.
Kat Landon’s life has just gone up in flames. Framed for treason, betrayed by those she trusted, and hunted by people who won’t stop until she’s dead, she has no choice but to go on the run. A career spent sacrificing everything for MI6 has made her a survivor, but this time, it might not be enough.
For years, she’s fought her feelings for Leo—convinced a relationship with him could never work. But when her enemies close in, there’s only one man she trusts.
Leo.
With danger stalking their every move and passion burning hotter than ever, Kat and Leo will have to fight not just for their lives—but for the future they never thought they deserved.

The Gentleman is the sixth and final book in the Guardsmen Security Series. It can be read as a standalone but for maximum enjoyment the series should be read in order.