Straight-arrow

n.thoughtful, kindplayful

What it means

A person known for honest, conventional, and dependable conduct.

Reach for it when using informal praise for integrity when rule-following is relevant but not blind

Colleagues trusted the straight-arrow to report the accounting error immediately.

playful
goody-goody
when excessive virtue is being mocked rather than praised
not when Goody-goody is dismissive and should not replace sincere respect.
everyday
reliable person
when dependability is the useful fact
not when Reliability alone does not establish honesty.
professional
honest broker
when impartial trusted dealing between parties is the role
not when An honest broker mediates interests rather than simply following rules.
professional
principled person
when choices follow clear ethical commitments
not when A principled dissenter may break conventional rules that a straight-arrow obeys.
formal
upright person
when moral integrity deserves direct praise
not when Upright is more elevated and need not imply conventional behavior.
literary
square dealer
when fair dealing in an older idiom is intended
not when The phrase is dated and narrower than general character.

known as a straight-arrow
She was known as a straight-arrow who documented every exception.
real straight-arrow
The real straight-arrow refused both the bribe and the flattering favor.
straight-arrow reputation
His straight-arrow reputation made the undisclosed conflict especially surprising.

Straight-arrow is informal and can imply rigid conventionality. Honest or principled is better when constructive rule-breaking matters.

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