Uh...just because they have the same hair color and are a bit crazy when they get angry, doesn't mean they are related. x''D And it would be so boring if they were, that's why no one said anything. This is Gintama. We don't need to see Gintoki's dad, what I want to hear more about is his sensei.
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Besides Gintoki is an orphan isn't he? His sensei found him...