There is no such thing under the law as "character assassination" under the law. The concept is a purely rhetorical one with no legal content. There only only "defamation" which is a form of lying about someone in a manner that damages their reputation. (The U.S. is much more hostile to defamation lawsuits than many other countries such as the U.K. and France.)
A small number of U.S. states have recognized a privacy tort known as "false light" where statements imply or state something that is misleading and intentionally causes emotional harm, but two-thirds of states have held that this violates the first amendment and rejected the tort, and many other states have tough restrictions on the tort due to first amendment concerns. There is a split of authority between the federal circuits in the U.S. over whether these torts are constitutional. The 10th Circuit, for example, has limited torts like false light to the same extent as defamation lawsuits, while the 5th Circuit, for example, has allowed such claims.
It is also possible to contractually agree not to say anything disparging about someone whether it is true or not, and if such a contractual agreement exists (or is implied in law from a relationship such as serving as someone's lawyer or public relations officer) there could be liability for breaching that contractual relationship.