Theo Ruiz is a lieutenant at Station 19 who used to be captain in his past but had a difficult time after the death of Michael, Travis' ex-husband, with whom he went to fire academy with and was best friends with.

He and Travis were able to put their past issues aside when Theo transferred to Station 19 after working alongside Andy at Station 23.

Theo begin dating Vic, and the two got serious, but some issues between them have since pulled them apart.

Theo temporarily served as Station 19's captain when Beckett was on leave, but his methods had him butting heads with many, especially Andy, and the others didn't trust his judgement.

He's resentful that Andy has been named captain.