Russian Beast by Aislinn Kearns

Alexei: Strong. Silent. Russian.
Alexei has lived a life off the radar since he arrived in the US years before. Until he meets Evie—the woman he can’t get out of his head.
Having escaped Russia—and a brutal father—when he was barely out of his teens, Alexei now earns his living in illegal fights. There, his size and strength are an advantage, not a curse. When Alexei rescues his neighbour from her violent ex, he sees for the first time that he can use his power for good, and becomes determined to protect Evie from her demons.
Evie has avoided men since she escaped her ex, focusing on the final months of her social work degree. Paranoia has gripped her, and she can’t be sure whether the shadow she thinks is following her is really there, or only a figment of her imagination. But when Alexei barges into her life, she has to decide whether she can place her trust in him, or whether he’s just another man who uses violence to get what he wants.
When Evie convinces Alexei to teach her to fight, she must learn to trust her instincts, to trust him. If she doesn’t, her ex will shatter everything she holds dear, and she’ll risk losing Alexei forever…