Final answer:
Antoninus Pius was Hadrian's main heir and became emperor after Hadrian's death. He continued Hadrian's policies and ruled until his death in 161 CE.
Step-by-step explanation:
Hadrian's main heir who became emperor was Antoninus Pius. After Hadrian's death in 138 CE, Antoninus Pius succeeded him as Roman Emperor. Antoninus Pius continued many of Hadrian's policies, including the construction of various public buildings and the maintaining of Roman stability. He ruled until his death in 161 CE.