Patient portal messaging has become a primary channel for asynchronous clinical communication, it spans a wide range of content, from symptom reports and medication concerns to administrative requests. Despite this volume and diversity, there is no formal representation for what a portal message contains: no vocabulary for the clinical and administrative events it describes, or for the attributes of those events that the patient has actually disclosed. Without such a representation, it is difficult to systematically analyze portal communication, assess message completeness, or build downstream tools that depend on structured input, such as automated triage, response drafting, and follow-up question generation. A clinical event schema, grounded in real portal messages and reviewed by clinicians, would provide this missing foundation. We introduce a clinical event ontology for patient portal messages, containing 8 event types and 70 roles that span clinical content (symptoms, medications, diagnostic tests, treatment responses, patient history) and administrative content (medical needs, logistics, social factors). The ontology was developed iteratively in collaboration with clinical expert and human evaluation. As a downstream application, we use the ontology to characterize the event types and roles most frequently sought in clinician follow-up questions, which provides insight of what clinicians ask about when reading portal messages.