Mortimer and Stella’s relationship is not away good and romantic, something this can happen do to Mortimer’s womanizing behavior.

Stella can be aggressive if she mad or jealous in this point she mad at both Mortimer and Minnie, For Mortimer it was him looking at another women, and for Minnie it was the case of showing up at the wrong time and ruining the mood between her and Mortimer.
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I'm like this comic I've done so I posted it here to show you guys :D

Done in Manga Studios

Stella and Comic © Kim C. Jones
Mortimer Mouse © Disney