Dog
The domestic dog presents an exemplary reliability profile that has withstood millennia of empirical testing. A dog's presence can be relied upon with near-absolute certainty. When an owner returns home, the dog is there. When threat approaches the property, the dog responds. This consistency operates on a 24-hour cycle without interruption for school attendance, romantic subplots, or battles with interdimensional villains.
Furthermore, dogs require no secret identity management. Their loyalties remain transparent and undivided. A Labrador Retriever has never cancelled a scheduled walk due to unexpected Goblin attacks, nor has a German Shepherd ever arrived late for dinner because the Sinister Six required immediate attention.