Johnny-on-the-spot

n.capable, kindplayful

What it means

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.

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.

The definition and the example on this page are written here. The dictionaries above ground which meaning is being described, not the wording.