Final answer:
Lucius Verus was co-emperor with Marcus Aurelius and managed crises like the Tiber flood and subsequent famine. Marcus Aurelius, known for his Stoic philosophy, was challenged by military campaigns and a plague. The decision to name Commodus as successor marked a leadership decline.
Step-by-step explanation:
The co-emperor referred to in the question is Lucius Verus, who ruled alongside Marcus Aurelius from 161 to 169 A.D. Lucius Verus's co-reign is known for his management of various crises, such as the flood of the Tiber and the subsequent famine.
Although Marcus Aurelius was indeed the more influential and philosophically renowned emperor, Lucius Verus held significant authority during his time as co-emperor. Marcus Aurelius is remembered as the last of the Five Good Emperors and for his work Meditations, which is central to Stoic philosophy.
His reign was marked by military campaigns against Germanic tribes and the infamous Antonine Plague, which Lucius Verus died of in 169 A.D. The later decision by Marcus Aurelius to name his son, Commodus, as his successor marked the decline of effective leadership, as Commodus's reign strayed significantly from the principles and administrative skill of his father.