What logical fallacy is this

To begin, my opponent says they have your best interests at heart, but how can that be true when they are lazy and unpleasant?

The logical fallacy in this statement is known as ad hominem. It is a fallacy where instead of addressing the argument or claim being made, the person's character, behavior, or traits are attacked. In this case, the opponent's laziness and unpleasantness are used as reasons to dismiss the notion that they have the best interests at heart.