Bowser's personality is explored most extensively in the various RPGs, starting with the first one, Super Mario RPG: Legend of the Seven Stars, in which Bowser loses his castle to a greater villain. As well as the grief he expresses over his castle, Bowser is shown to be as sad as the other playable characters about Geno's body breaking down, and at one point, his boasting takes the form of a haiku. As in later RPGs, however, most of Bowser's actions in the game are self-centered, with his motivation for joining the heroes solely being to reclaim his stolen castle and prevent another villain from taking over the world that he himself desires.
Bowser has never shown remorse for his troublesome actions and the havoc they often cause, but he is often saddened by his own losses. In Super Mario Galaxy, his only regret is his failure to create his own realm, and in Mario & Luigi: Bowser's Inside Story, he is upset about having to damage his own captured castle as he tried to reclaim it from Fawful.
While his range of emotions varies from game to game, it is his nearsightedness, frustration, anger, and contempt that typifies Bowser in all his appearances, whether he is a reluctant protagonist or a straightforward villain who cares only about conquering lands and destroying his opposition.