Broken series (1-3) by Skyla Madi

Broken (Broken, #1)
Emily's life sounded more like the punch line to a joke than an actual turn of events.

... a student nurse walks into an illegal, underground fighting club...

Emily Sheppard didn't have much to live for. As a foster child, her life was anything but glamorous and her daily commute through New York's underbelly reminded her of that. Nursing was her life, her only way to help others in need.

Then her pride took it from her.

Jai Stone wasn't important. The train he rode around New York never let him forget that. Jai had fights to fight and a brother to save, but one random night slipped an innocent into his grasp.

Now they're trapped together.
A fighter and a nurse.
... and it all comes down to one truth.

FIGHT OR DIE.
Twisted (Broken, #2)
Does a kitten have what it takes to become a lion?

The underground fighting ring is a brutal game and Emily Sheppard is at the center of it all. With only two choices, Emily is torn between betraying the man who saved her life or accompanying him down the road to certain death.

Jai Stone is on a mission to save his brother. The road that was once clear and simple has become murky and filled with uncertainties. Can they make it past round two? Or will Skull kill them before the round even begins?

Once again, the fighter and the nurse fight for their lives in a deadly world of murder and deceit.
Damaged (Broken, #3)
In the aftermath of an escape that almost claimed their lives, Jai and Emily are on the run. While Emily uses the time to recover and heal, Jai prepares for war.

His resolve is unwavering.
His thirst for vengeance unquenchable.

Can Jai and Emily find Joel in time? Or will Skull catch them and make them pay for their betrayal before Joel's whereabouts is discovered?