Bess, Soul Nourisher

If a creature has a characteristic-defining ability that sets its power and toughness, indicated with a */* or similar in the power and toughness box, that ability is taken into account when determining its base power and toughness.

Normally, a creature's base power and toughness are the power and toughness printed on the card or, for a token, the power and toughness set by the effect that created it. If another effect sets a creature's power and toughness to specific numbers or values, those become its base power and toughness. If an effect modifies a creature's power and/or toughness without setting them, that is not included when determining its base power and toughness.

Some creatures have base power and toughness 0/0 and an ability that gives them a bonus based on some criteria. Those are not characteristic-defining abilities, and that ability doesn't change its base power and toughness.