A first responder is a person with specialized training who is among the first to arrive and provide assistance or incident resolution at the scene of an emergency. Wikipedia

The first medically-trained responder to arrive on scene of an emergency, accident, natural or human-made disaster, or similar event. Such people may be police or other law enforcement, firefighters, emergency medical services, or lay rescuers. OmegaWiki

A member of an emergency service who is first on the scene, or among those first on the scene, at an emergency. Wiktionary

A member of an emergency service who is first on the scene at an emergency. Wiktionary (translation)