I am looking for a word similar to hometown hero, or someone who willingly helps those within their community who are in need/danger. This is different to helping as a part of a job (eg. doctor, social worker), but rather helping because they choose to. Something similar to an idol, Samaritan or savior but without any religious associations, if that makes sense.
You seek something comparable to hometown hero. Hero suggests someone from the town who returned from war or sports or whatever as a hero, or perhaps once helped in a flood or other disaster. "Community stalwart"might serve your purpose better.--A loyal, reliable, and hard-working supporter of or participant in an organization or team," (ODO), the team being the community. Johnny-on-the-spot does not cover the idea of continuous service, but may relate only to a one-time event.
A person who is on hand and ready to perform a service or respond to an emergency.
a person who is on hand to perform a service, seize an opportunity, deal with an emergency, etc.
E.g. I'll be Johnny-on-the-spot all right, and have it ready for you.