Final answer:
Romulus shares a significant lifelong relationship with his twin brother Remus, as they were both suckled by a she-wolf according to Roman legend and founded the city of Rome together before Romulus eventually killed Remus.
Step-by-step explanation:
The individual who shares a lifelong and significant relationship with Romulus is his twin brother, Remus. According to Roman mythology, after being abandoned and left to die by a jealous king, they were rescued and suckled by a she-wolf. Romulus, often considered as the namesake of Rome, is famous for founding the city and establishing monarchical rule. His reign was followed by six other kings until the establishment of the Republic in 509 BCE.
Despite the legendary nature of the story of Romulus and Remus, it has come to symbolize the spirit with which Romans saw themselves, as descendants of a line starting from two infants raised by a she-wolf. This story, while part myth and part symbol, represents an important aspect of Roman cultural identity, embracing the qualities of bravery, force, and power through its narrative.