Harmless, innocuous, innocent, inoffensive, unoffending are comparable when meaning not having hurtful or injurious qualities.
Harmless may be applied to whatever seems incapable of doing harm or to what in comparison with others of its kind or with members of other kinds is free from all power to hurt or injure.
Innocuous differs almost imperceptibly from harmless in meaning. It is, however, the term preferred in technical use to specify the absence of the properties or substances in some members of a group that render other members of the same group harmful or dangerous.
In extended use, therefore, it is often applied to something that seems harmless only by comparison or that is harmless to such a degree as to merit contempt.
Innocent fundamentally implies guiltlessness and suggests an accusation which can be or has been disproved, but the term has become applicable to actions and qualities and, in this sense, carries the implication of absence of all potential evil especially in intention or in capacity to do harm.
Inoffensive implies harmlessness in a degree that is almost excessive or pitiful; it is usually applied to persons and animals.
Sometimes it suggests an incapacity for offending and in this sense equals unoffending, particularly when it is applied to inanimate things.