Aster-Alarie
This book used to be titled Frienemies but was renamed due to me being stupid and not knowing how to spell English words.
Thirty year old assassin, Ali, has been trying to track down her step brother for six years. She wants to take back what he stole all those years ago but while doing a favor for her boss she stumbles upon a Kingsman agent. She's familiar with their organization but neither one recognizes each other and she doesn't want to be dragged into involvement with them. However the agent is interested in her because of her unique personality and the way she acts.