One thing I’m really courios about is why radizen calls Marucho Marucho-sama isn’t that reserved for villains or something
no, it's an honorific that denotes a high level of respect/politeness and difference in rank. that's probably why you've heard villains using it (as in, minions referring to their superiors with "-sama") but lots of other characters use it, too.
kato, for example, refers to marucho as "marucho-sama" because he's the young master that kato serves and marucho also refers to his parents as "otou-sama" and "okaa-sama," which shows a greater level of formality toward his parents compared to other characters (eg. dan calling his father "tou-san" or mira and runo calling their fathers "papa.")
unlike marucho's other partner bakugan, radizen speaks in keigo (polite language) so that's likely why he refers to marucho with a very respectful honorific, similar to how hawktor referred to shun as "shun-danna" (master/boss) or how skytruss and orbeum call him "shun-dono" (respectful honorific that is below -sama.)
some partner bakugan address their battlers very casually while others are very polite. it tends to either show through the bakugan's "background" (archaic like samurais/ninjas or the like) and their use of language (eg. kansai-ben vs keigo)
