Sirius

 is the warlock patron of Zahra Hydris. Sirius has been mentioned but has not appeared on Critical Role.

Background
Sirius appears to be closely associated with the moon(s). Zahra's father kept her caged indoors as a child, and when her father died and she went outdoors for the first time, she looked up at the moon, and it sang to her and saved her life. Zahra used a moonstone as an arcane focus and forged a Moonhammer for Grog.

Zahra asked for, and received, Sirius's boon in the ritual to resurrect Velora Vessar.

Relationships
Very little is known about Sirius, but he supposedly entrusted Zahra Hydris with watching over and judging Vox Machina.

Trivia

 * His description and type of warlock patron indicates he might be one of the star spirits, entities once connected to the moon goddess Sehanine.