Ekko could have saved his friend

In the story it pretty much goes that he marks a point in time and then is able to go back to that point over and over again. look for example at the "seconds" video. https://www.youtube.com/watch?v=FtEpp9lkfxA

No matter when he rewinds,he always goes back to the moment where he pulled that string.Meaning that he can't go further back than to this point.

It is possible that the way he uses the time traveling ingame is an alternative use for his device(which comes at a large cooldown,hence why he couldn't use it) or that it is simply modified to fit the game a bit better(rewinding over and over again would be really op and if you had to pull the line first before returning,then that would feel rather clunky)

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The device doesn't seem to follow the rules it does in game, which is 4 seconds back. Unless he was able to traverse the whole city in 4 seconds.

I thought he did? I mean, I think I remember him saying '2. something seconds, best time yet'

Go back and rewatch Seconds. The Z-Drive, when activated with the pullcord, basically sets a temporal anchor at that exact moment in time. Whenever Ekko reactivates the device with his wrist switch, the Z-Drive pulls him back through time to the anchor point. Presumably there are limits on how far back Ekko can go, but it's at least long enough for him to reach Ajuna - which took him around 5 minutes at his slowest, according to the comic.

In the Ekko comic, Ekko didn't pull the cord until after Ajuna had already been shot. Therefore, no matter how many times he went back - no matter how fast he was at getting to Ajuna's side - it was already too late. The only reason he kept going back was to find the one who killed his friend and get revenge.

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Did you actually read the comic?

Yeah but I don't remember it