A characteristic is a fact which tells us something about the nature of things. If a fact is sometimes connected with a things and sometimes not, then it is not a characteristic and will not help us very much to understand the nature of that thing.
Heat, for example, is a fact. Heat is not characteristic of water, as water is not always hot. the heat of water depends upon other factors like sun or an electric stove. But heat is a characteristic of fire because fire is always hot and heat of fire does not depend on any other factors.Heat is always connected with fire and tells us something about the nature of fire.