Stubborn • Instinctual • Informal • Simple-minded • Emotional • Gullible 

Bright is a very instinctual person, primarily going by his gut feeling when making decisions. He learned that if something gives him bad vibes, he should stay away. His instinct has saved his hide more than a couple of times so he learned to trust it above all logic and emotion. His ability to sense when things mean bad news is almost like a superpower.
He can not be expected to handle anything with care, things in his possession will end up with dents and scratches no matter what material. He doesn't care what others think about him. He's very non-chalant about anything he might have done wrong. Partly because he does not care, partly because he simply does not understand.
Bright isn't the smartest, but at the very least he's aware he's not the sharpest tool in the box. He's simple-minded, not able to follow most logical deductions, but he doesn't pity himself for it. In his eyes, that's just how he is and he can't change it. He's very content with other people (specifically Mayhem) doing the thinking for him. Due to this he's very gullible, believing most things he's told just because he doesn't know better.
He's very outward his emotions. He could not hide how he feels to save his life but he does not try to begin with. His emotional state is like an open book displayed on his face. Like his emotions, he's also very open with his opinions.
Bright is a very casual person with no concept of formality. Despite his lack of social awareness, Bright is a very emotionally intuitive person. He can read people extremely well, perhaps precisely because he is so open with his own emotions.
He doesn't have a malicious bone in his body. He could never bring himself to hurt someone on purpose and he's fundamentally not an aggressive person. That does not mean, however, he is opposed to violence. While he isn't the one to start a conflict, he wouldn't hesistate to end it. If it's to protect someone he cares about, he would do it by any means necessary. Bright is a loyal friend and a reliable shoulder to cry on. Due to his empathetic nature, he enjoys being a beacon of optimism for others and if he can tell that someone needs it, he'll make himself appear more joyful than he actually is to make them feel better.
He's stubborn as a mule and does not like being told what to do by anyone. Luckily, since he's so stupid, you can make him think something is his idea and then he'll do it.