non-assertive
/ˌnɒnəˈsəːtɪv/adjective
1 Grammar. Not expressing assertion, not making a definite or positive statement; not expressing one unambiguous meaning.
2Of a person, a person's character, an organization, etc.: not pushy, intrusive, or self-assertive; timid, self-effacing.
Origin
Mid 19th century. From non- + assertive.