For example, notice how the bonta-kun suit changes its facial expression from happy to angry. When Souske puts on the suit, it becomes Souske himself, so therefore, the angry facial expression appears and so does the scar.
That's my interpretation of the scar anyway.
Why does Bonta-kun only say "Fumoffu"? Initially, Sousuke installed a voice changer in the Bonta-kun mecha to disguise his voice with, but due to some unknown mechanical errors, everytime he turns off the voice changer, everything else stops fucntionning and the whole unit just shuts down. So, the only way to keep things running is to keep the voice changer on; that's why all we ever hear from him is "FUMO FUMORURU FUMOFFU!".
Why doesn't Sousuke just fix the Voice Changer? When Sousuke finished modifying the Bonta-kun suit, he discovered many errors in the programming such as inaccurate aim. There were still many bugs to fix and the Voice Changer was just a minor problem. I guess the reason why he never fixed it is because he finally got used to it. It also sounds so much more cooler than normal talking. Don't you think so?
If my favorite character is Bonta-kun, doesn't that make Sousuke Sagara my favorite character too? It depends on your point of view. Some people like the original Bonta-kun of Fumo Fumo Land, and some of us (MOST OF US) like Sousuke's mecha version of Bonta-kun. That didn't seem to answer the question, so If you want Sousuke to be your favorite character too, then yes. If you hate Sousuke, then no.