Tengen Toppa would not be half the anime it is without Kamina, the thread that holds the entire fabric together. True, Simon is in fact the central protagonist, but the star quality Kamina brings to the plot is unrivalled. He is cocky, charming, invigorating, tragic, and fearless, a hero whose personality overshadows everything. In terms of charisma, he is probably outdone only by Goro Honda from Major.
Simon, on the other hand, did not show his mettle until quite late into the series. Much of the beginning and middle episodes were used to peel off the layers of his innocence one by one until what was left was the determined and, disappointingly, semi-charismatic core. He rode a powerful mecha and made some great speeches, but in the end I found him less exciting to watch than some of the others.
Those others would include Yoko the sassy sharpshooter, who unfortunately became the sole victim of the series' ecchi leanings, Rossiu, a former priest's apprentice whose development was one of the most complex, Nia, a charming abandoned princess looking for her identity, and Viral the beastman, who turned out to be far more than just an amusing recurring antagonist. Actually, the high number of characters that were developed is commendable considering the show was only twenty-seven episodes; getting attached to several of them was easy.