
Being in contact with Athena's cosmos since early childhood it's what have the protags their main character powers.
Basically it unlocked their potential, so Seiya and co are capable of improve their abilities beyond what a human can, and getting pretty close of the likes of Gods, but only while Athena 'buffs' them with her cosmos. This is why sometimes they can punch God in the face and sometimes gold level saints kick their collective asses.
This doesn't mean that they suddenly become invincible while Athena helps them out ; they have to grow their power with effort and training like any person, but this unlocked potential makes them develop faster and in emergencies allow them to push their bodies far beyond what a normal person could, and cling to life when they should have had died.
This also isn't limited to the main gang, the secondary bronzes could also theorically do it, but since the power scale grew so big so quickly they couldn't catch up fast enough to be useful. RIP.
And since this happens for being near Athena while a child develops, Kiki will grow up to have unlimited potential as well.