Ekko is a homophonic pun on Latinate English echo /ˈɛkoʊ/;
from Ancient Greek ἠχώ /ɛː.kʰɔ̌ː/ "sound" (var. ἠχή /ɛː.kʰɛ̌ː/ & ἦχος /ɛ̂ː.kʰos/) < PIE*sweh₂gʰ-:
While Ekko himself does not have a sound theme nor does he take after the Nymph from Greek mythology, the idea is that his past selves from his constant rewinding of time to achieve the results he desires serve as an echo of split timelines he left behind (as evidenced by his holographic projections in Parallel Convergence and Chronobreak)
Besides the proverbial-and-literal 'Ekkos of the past' his name having a double K references the Rewind symbol (<<)
Ekko is a 16-17-year-old teenager, younger than Jinx and Vi.
Like Jinx and Vi 'Ekko' is not his actual name but rather what his friends call him.
Ekko is basically a gifted, unrefined genius. He managed to create the Zero-Drive and the Timewinder simply through trial and error, and as a result split the timeline to fuel his perfectionist side (from finding an enemy's weak spots to rewind time over and over to save his friend Ajuna in 'Chronobreak') 
This rewinding and his intellect allowed him to take down a Vigilnaut (the techmaturgically augmented goon he fought in 'Seconds') 
Each rewinding makes him a bit older than he's supposed to be (for comparison, if Ekko rewinded 200 times in 30 seconds he'd age 1 hour and 40 minutes) 
Ekko's facepaint and Zero-Drive are both shaped after an hourglass, an expression of his personality as well as the symbol he adopted for himself.
He displays the same Zaunite street fashion Jinx and Vi do (with Ekko's neckpiece being identical to the hextech bolts on her gauntlets)
Ekko's 'bat' is a clock hand he stole from a Piltie clocktower (as evidenced by the Roman 'XII' at its base)