By Season 6, Stella and Severide had agreed to keep their relationship platonic. However, their plan backfired when Stella was evicted from her apartment and needed somewhere to live. Severide eventually offered his former love a spot in his guest room, further complicating their dynamic.
In one episode, “Down Is Better,” Stella not-so-subtly hinted that she wanted more from Severide when she drunkenly kissed him. To her (and fans’) surprise, Severide denied her advances and claimed they weren’t in the right space for a relationship.
After getting rejected, Stella moved on and started dating other people. However, by the Season 6 finale, the couple reunited once again. Severide made it official this time by asking Stella to be his girlfriend.
Source: https://www.distractify.com/p/kelly-severide-and-stella-kidd-chicago-fire-relationship-timeline
