I really don't remember much about it, it was about two aliens, one red and the other one is green. They appeared above a city and started to fight. I not really sure if thats right, so don't get annoyed.

I guess you can hi-jack someones idea, but you'll never understand their plan for its success.
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not much of a description to go by, but it sounds like Karakuridouji Ultimo

Ultimo maybe.. although they are more like robots then aliens.. I've only read the first chapter, so I'm not sure.

Does the walker choose the path, or the path the walker?