First of all, your characters probably won’t get into a relationship with someone actively toxic. So the toxic character will probably be nice until they’ve gained the trust of whoever they’re targeting.
They might be a charming person who’s toxicity is not obvious.
They might be charming to everyone except the person they’re targeting
They might only target people who they can manipulate.
They might not realize they’re toxic because of how they rationalize their behavior.
When confronted on bad behavior, they might actively refuse to apologize, put the blame on their victim, and/or publicly humiliate them.
Your character, the non-toxic one, might not even realize they’re involved with someone horrible.
Your toxic character might lecture your non-toxic character on every little thing they do wrong.
They might try to control your non-toxic character, and this doesn’t necessarily have to be something big. If a toxic character doesn’t like your other character to be playing a game of cards, they might convince your character to stop.
A toxic character might seem confident
When long term abuse occurs, love almost always coexists, and it could be hard to realize how bad the relationship is because there are so many good times with the bad.
Physical abuse doesn’t have to occur, but it can.
Is physical abuse does exist in the relationship, getting out of the relationship could get your character killed.
If there is no physical abuse, your toxic character has relied solely on emotional abuse.
The toxic character might use the silent treatment.
They might guilt trip your non-toxic character.
They might destroy opportunities for your character by coming up with reasons why the opportunity is a bad idea.
They might tell your character they’re not good enough for an opportunity.
Sometimes, an absuive person will wait until someone is legally bound to them before they show their true colors.
The toxic character could be a family member (parent, sibling, spouse).
If your non-toxic character realizes they’re involved with someone toxic, they might forget the reasons why they’re toxic because the toxic character can be really nice at times.
And finally, If you have a toxic character with stalkerish, creepy, possessive, and obsessive behavior, do not romanticize it.