i will use adult adorabat for this idea
the pure heart makes adorabat go crazy in one episode, forcing her to transform into a semi monster form, she however keeps her intelligence, and knows that she has only 5 hours before her body achieves her "full potential", mao mao and badgerclops manage to defeat her just in time, and adorabat remembers everything she did and feels guilty because of it, mao mao puts her in jail for 5 minutes to teach her a lesson, adorabat says that she can use this semi monster form at will but only for 10 minutes or she may end up achieving her full potential.
in another episode badgerclops ask "hey adorabat, what did you mean with "achieving your "full potential""?
"oh! its something that my mom told me, it means that- (they get interrupted by a monster, they fight and win, at the end adorabat grabs mao mao and badgerclop's hands) you see, my mom told me that i will achieve my full potential in maybe... 5 years? and that the "full potential" is a stronger form, way stronger than any monster we have fought so far, including the giant orangusnake"
MM: thats nice adorabat, now can i get my hand back?
BC: yeah, let my hand go dude
AB: ... i will most likely go berserk after that transformation and if that happens... -puts mao's and badgerclop's hand on her neck- ...i want you both to kill me.