Johnny-on-the-spot

A person who is present and ready to help exactly when needed.

Reach for it when praising reliable availability in colorful informal language

Tariq was the johnny-on-the-spot when the sound system lost power.

• playful

  lifesaver

  timely help deserves grateful exaggeration in casual speech

  not when Lifesaver can overstate a routine favor and is literal in emergencies.

• everyday

  go-to person

  the group regularly relies on one person's know-how

  not when A go-to person may advise remotely and need not arrive quickly.

• warm

  reliable helper

  dependable assistance matters more than colorful phrasing

  not when Reliable helper omits the crucial idea of being present at the right moment.

• warm

  right-hand person

  trusted ongoing support to a leader is central

  not when Right-hand person implies hierarchy and a continuing relationship.

• professional

  first responder

  trained emergency help is literally involved

  not when First responder is a defined public-safety role and must not label an ordinary volunteer.

• professional

  troubleshooter

  the person diagnoses and resolves unexpected faults

  not when Troubleshooter emphasizes technical problem-solving, not simple timely service.

• absentee
• bystander
• no-show

• reliable johnny-on-the-spot

  A reliable johnny-on-the-spot brought spare cables before rehearsal began.

• local johnny-on-the-spot

  The local johnny-on-the-spot knew exactly whom to call about the leak.

• ever-ready johnny-on-the-spot

  Their ever-ready johnny-on-the-spot found chairs for every unexpected guest.

Johnny-on-the-spot is dated and gendered; go-to person is clearer in most current workplace writing.