Roger Grant was Stella Kidd's ex-husband.
Grant was first seen in Season 4 after Stella joined Firehouse 51. He relied on her for helping him with his anxiety, which some of the others saw as a lack of dependance and the fact that he was still in love with her, which she denied.
At the end of Season 4, he was sent to a psychiatric hold but escaped in 5x01 and went after Stella and Severide, who Stella was unofficially dating at the time. She is worried but he tells her he is staying with a friend and is okay. However, he stalks her with a knife until he finally comes out and is caught by Severide outside Molly's. Severide ends up stabbing him in the neck when Grant threatens them.
Stella decided not to press charges against Grant twice, which angered Severide and ultimately led to their breakup as it was clear she still cared about him too much.