Dark skin? Light hair? Time powers through technology?
http://vignette2.wikia.nocookie.net/skullgirls/images/c/cf/20130325090055-DLC_Isaac.png/revision/latest?cb=20130325161017
Joking aside: ZAUN!
Infrastructure! Architecture! The city seems definitely layered, the place Ekko and his aggressor are standing on is suspended on thin panels. Cybernetic augmentation, if Vi/Urgot/Singed Jr. is anything to go by, seems to be more commonplace than previously thought. Unless the unnamed character is one of Viktor's Glorious Evolution, which I doubt on the basis of aesthetics, mechanical sophistication, and personal bias, people are slapping gears and pistons on themselves with surprisingly little reservation, which also makes me wonder why Viktor's goals are considered odd, though that may be by the world at large rather than just Zaun.
Another thing about our unnamed assailant; take a look at the face on the mural's far right, then look at Big McLargehand's hair. Seems rather familiar to me. Could be the same guy. Also the broken goggles, which are with their rightful owner in the mural, something Ekko willingly turns back time to preserve. This makes it clear to me the painting, and by association the people in it, are important to him. Perhaps they were part of a gang that has been whittled down to just him, and Big McLargehand is a former friend turned human experiment?
Also also, Ekko's name may be more than a play on his abilities. He is in obvious pain after each temporal reset, yet has no visible injuries. This could be graphical limitations, as changing his model would have cost more time, effort, and money, but I think it may have something to do with the nature of his time travel. He clearly remembers previous events, despite them technically never happening after he alters the future (paradoxes, yo), but is physically no worse for wear; perhaps the pain and memory are an "ekko" of his time travel, like phantom pain for body and mind.