Final answer:
Antoninus Pius was the main heir of Hadrian who became emperor, succeeding him. He was adopted by Hadrian, which followed the trend of adopting capable individuals for succession rather than relying on natural-born sons.
Step-by-step explanation:
The main heir of Hadrian who became emperor was Antoninus Pius. After the reign of Hadrian, Antoninus Pius was chosen as his successor and thus became emperor. During this period, emperors were often selected based on their abilities rather than bloodline, with Antoninus Pius being adopted by Hadrian. Marcus Aurelius, the last of the Five Good Emperors, was raised with the role of emperor in mind and later became known for his Stoic philosophy and leadership. Aurelius made the unprecedented decision to name his biological son, Commodus, as his co-emperor and successor, which marked a departure from the practice of choosing a capable adopted heir like in the case of Antoninus Pius, who was adopted by Hadrian and became his heir.