An attorney would need more information to be able to give you expert advice. However, I can share some basic information that may be helpful to you. If a coworker is harassing you on account of your age, race, color, national origin, gender, religion or disability, then your employer would be obligated to take appropriate "corrective action." Corrective action could include disciplining the harassing employee, providing training regarding appropriate workplace behavior, transferring an employee, etc.
If two employees are having a personality conflict, the employer is not legally obligated to do anything about it. It makes good business sense for the employer to address the problem, but they would not be violating the law if they simple ignored it.
How do you know that the other employee was not disciplined? Most employers are very tight lipped about the discipline that they hand out. The coworker could have been disciplined and you might not ever know about it. Over the years, I've heard many, many individuals claim that the coworker was not disciplined, and they've very often been wrong.