It's a character whose presence "warps" the other characters around them, causes them to adopt unconventional behavior or roles in reaction to the character, but there are instances where that can be done credibly and justifiably.
Indeed. And we can see that in many episodes of TV shows that introduce past lovers/teachers/parents etc. Was Spock only
being affected by spores in "This Side of Paradise" or was Leila Kalomi also having an effect, etc? All part of the fun of introducing a guest character in the first place.