Status: Siblings; Love Each Other, Former Allies (She was secretly working against him), Both are archangels; Ended (Uriel died).
This is the relationship between the sibling Archangels, Michael and Uriel. Michael is Uriel's younger brother. She loves him deeply and seems to be willing to stand with him against Gabriel. In Michael's younger days as the Flood, Uriel would act as the voice of reason between Michael and Gabriel.
However, it is later revealed that she is standing with both brothers (to deceive them) in order to bring peace on Earth.
In season 2, Michael learns from Gabriel that Uriel was killed in an airstrike, orchestrated by Claire Riesen. Michael uses his violence as a reaction (against Gabriel) to her death, clearly grieving her.