Why yes, @Bridalton i have watched the anime. Otherwise i wouldn't have known about this. Commonly, when a letter is enclosed in a heart, there tends to be romantic/affection feelings for whom the letter stands for. Thus, if the "k" stood for king, that would not make any sense. Due to the fact that the word "king" does not have any emotions tied with it in the mind of Katsuki Bakugo. Also i would like to point out that the fantasy AU that the picture depicts does not have any association with the world in which that Bakugo has picked his hero name. Thanks!